I've accumulated a collection of supports, including a heavy 055 Manfrotto, an old Manfrotto 134 monopod, and a very lightweight Manfrotto BeFree, plus a couple of much older bits and bobs that I never use. I want to sell some of these on and move to one support, a lightweight but very strong tripod, with a detachable monopod, and an Arca Swiss compatible head. I only shoot stills, never video, and my heaviest setup is a Nikon D5 with a 70-200 2.8 lens. I use my monopod very rarely, whenever I'm shooting sports and I've got a bad back. I use my tripod for leisurely landscape work, sometimes up hills and in other windy places. But, I'm also tinkering with astrophotography, so I want something that's rock solid, ideally carbon, with super strong leg locks that won't break after a few uses, and with a head that can tilt 90 degrees up to the sky. I've ruled out a gimbal head, I'm looking for a ball or a reasonably simple 3 axis. I don't think it needs to be damped, but it does need to be reliably strong. And I don't want everything to move when I press the trigger, like it does with the Manfrotto QR system. My budget is up to £150. I've looked at loads online, but they've all got heads which won't point straight up. Thanks!
Good advice, as far as it goes, from @Bobby2Shots, but budget aside, I'm not sure that even your desired specs are realistic, as insisting on a built-in monopod will severely limit your choices, and I can't recall ever seeing a built-in monopod arrangement on a tripod that I would call "rock-solid", usually they are to be found on much-lighter tripods. Limited choices means you'll be passing-up opportunities -- store sales of new or postings of used that might otherwise fit your tripod requirements (aside from price, which is unrealistic). And even if the monopod is only used occasionaly, if to use it requires semi-"disassembly" of the tripod, then that procedure is an impediment, and one is likely to use the monopod even less.
I think if you really need to keep expenditure very low, best bet might be to check estate sales, and photographic "swap meets", if there are any in your area, you might get lucky. Since you also already have some tripod experience, "hands-on" at such sales would let you actually evalute whether the item will meet your needs, and be better than what you already have by enough margin to be worth buying.
For what I would call quite-solid, yet not too heavy, I'd look at something in carbon fiber with thickest leg section around 36 mm diameter. One could of course go bigger, with 40mm dia legs, but weight is rising, and also probably collapsed diameter too, as many 40mm top-leg-tube tripods have a greater-diameter apex/spider. Or one could go smaller than 36mm dia legs. But I wouldn't go smaller than 31 or 32mm diameter for the top leg section. At least not my definition of adequately-solid.
I like Leofoto tripods, I think they are well-made (and I say that owning an old aluminum Gitzo, so I do have a standard against which to measure). Maybe not quite as good as Gitzo or RRS, and maybe it'll turn out that my Leofoto LM-364CL doesn't last as long as as those top brands, but I'm reasonably careful with my tripods, and I think the Leofoto will last as long as I will, and that's good-enough for me. (Plus I can't afford a Gitzo or RRS nowadays). Also, in the U.S., Leofoto repair parts, if ever needed, are easily available retail from LeofotoUSA.com, the official U.S. distributor. Don't see such parts easily availability for the other Chinese brands.
If in the U.S., I recommend the "Desmond Photographic" store on Ebay, I've bought from them, and ratings are good. Appears to be gray-market for Leofoto, but there isn't that much to go wrong on a well-made tripod such as Leofoto, so I'm not concerned if no official U.S. Leofoto warranty. Desmond sale prices do fluctuate, and on occasion sometimes go quite high, but the latter is probably a matter of complying with Federal Trade Commission rules (I think you can't legally claim a %-off if you don't occasional sell it at or near the higher price). Unfortunately, Desmond Leofoto selection nowadays is no longer as good as it was say half a year ago -- Leofoto is making some changes to its whole model line, many of the old models were/are being cleared-out, and some are gone.
Maybe look at the Leofoto LQ-324c or LQ-365c if you want a built-in (sliding between the legs) center column; maybe an LS-36_ or LS-32_ series model if you know you'll never want such a built-in center column, or an LM-36_ or LM-40_ series model if you want the option to later add such a built-in center column. (3rd digit in model number, to replace my underscore, is number of leg sections. Then add a "C" at the end (probably signifies carbon-fiber). Add an "L" after that for the few models also available in a long version).
But the LM- models are greater-diameter collapsed than either the LS- or LQ-series, because the LM-36_ series can use 75mm diameter video bowl accessories, and the LM-40_ series 100mm video bowl accessories. (You don't have to use a video bowl, you can use the included flat top plate).
For the LM- series, you can buy a drop-in center column, sliding or geared, but a not-insignificant extra expense to go that route. For much less, you can also buy accessory two-piece sliding carbon-fiber center columns that are screwed onto the top plate of the LM- (or LS-) series tripods and sit above it. Leofoto sells up-to 32mm diameter ones, but if you want more (potential) stability, you can get 36mm or 40mm ones from Artcise or Innorel. No personal experience with them.
EDIT: I see your currency is £ (didn't register before) so Leofoto via Desmond Photographic is probably not an option for you, unless you or a relative is visiting the U.S. (Don't know if Desmond ships internationally, but would add to cost, plus probably U.K. import taxes). Maybe other Leofoto gray market retailers in the U.K.? Thanks to the Tories and Brexit, your purchase options are undoubtedly less than when you were part of the E.U.