can anybody recommend a good inexpensive polarising filter for the canon stm 18-135 lens? Can this be screwed onto a uv filter or do I have to remove the uv first?
The company I work for sells a kit that includes an ND, UV and CPL for $14. I bought one and I've used all three filters extensively and they work well. Cheap filters don't necessarily mean bad images. The UV wars will rage on till the end of time, or at least the end of photography, but if you can't afford expensive filters and you don't mind a tiny loss of sharpness in your images, I say go for it.
Under favorable conditions, even cheap filters won't be a problem. The issue is when you are shooting in challenging conditions. This is where a cheap filter can easily ruin a shot.
You want to be careful anytime there is a light source in, or near, the field of view (such as shooting at the beach with the sun setting in the background). It can also be an issue when shooting products against an over lit white background. Shooting at night when there are lights in the image, or even when using a hair light or rim light.
If you are shooting in easy conditions, then a $14 filter is fine.
You can also use cheap filters with a very high quality lens. A $2,000 lens produces an excellent quality image. You have a lot of room to drag down image quality and still get good images.