Thanks for all the replies here. Really good advice and very helpful
In the end, I decided to go as wide and light as I could (also I have to be slightly realistic about costs, I come to realise!)
So, I hope you won't think I'm trolling when I say that I went for the OMD E-M10 with 9-18mm lens.
I appreciate I won't get quite the width or perhaps quite the IQ of the X-T1 with 10-24mm lens, but it is still going to produce some great photos - mostly probably better then the D90 with 10-20mm lens (which I was pretty happy with bar the 1.2 kg weight around my neck!)
My thinking:
X-T1+10-24mm = 440g + 410g = 850g
X-T1+10-25mm = £1049 + £849 = £1898
OMD E-M10 + 9-18mm = 396g + 155g = 551g
OMD E-M10 + 9-18mm = £529 + £479 = £998
That's 300g and £900 difference, which, for me and my travels, is hard to ignore.