My bike came with a cheap plastic bashguard - it broke and I need to replace it. However, I seem to be having difficulty finding them cheap anywhere, all I can find is high quality expensive ones.

I need a 4 bolt 42T bash guard, and I live in the UK. Can anyone recommend where to buy one?

Likely the issue you are having is that the piece that came on your crankset was part of the original package, and not an add-on guard like the true bash guards on the market. Those bash guards replace a chainring on the outside of the crankset versus bolting on to the outer chainring like your 42T guard likely did. My question for you is, do you really need the guard there? Aside from keeping your pants/calf a little cleaner, it's not doing much. The fact that it broke should tell you that it wouldn't be much help in a situation where a real bash guard would help.

If you truly expect it to function as a bash guard, only the more expesive ones, such as E13 will do.
What you describe as a bash guard may in reality be a chain guard. To be a real bash guard, it will mount directly to the spider in place of the outer chain ring. Did your original guard mount this way, or was it mounted to the outer chainring by means of small screws?

I have had two pairs of trousers wrecked because they have caught on the teeth of the outer chain ring, which never happened when there was a guard there, so I would like it replaced.

And it was mounted with small screws.

those chain guards typically come on cheaper cranks, and are rarely sold separately. You might be able to buy a whole new crank for a reasonable price though. Or, if you have a bench grinder and don't mind loosing your outer gear, you can simply grind the outer teeth off it. It works surprisingly well.

Maybe ya should just get these.

Trouser guard then, not bash guard. Bash guards, as the name implies, takes the hits from rocks, logs, etc. to protect the other parts. A flimsy plastic ring won't do much to accomplish that.

Seriously, what you're looking for is a plastic chain guard. They do come in different configurations regarding the mounting screws. I would check at bike shops, they might have something sitting around that would work. I found one for a fellow the other day on an old crankset we had removed from another bike-

https://www.bbgbashguard.com/Mountainbike.html

They ship world wide too.

If your main concern is your pantlegs, perhaps an SKS Chainboard is better suited for the task. SKS is out of Germany, I suppose you could source it in the UK without too much bother:

2 BBG Superlites on cyclocross bike.

If I were to use a BBG Superlite as a pants guard on my 42 ring, would I order a 42 Superlite or a 44?

42