I have just installed LAP GU10 LED Lamp 346Lm Cd 5W Pack of 5 for £ 19.99 from screw fix - they are dimmable ...
but , when dimmed they make a really loud buzzing noise .
Now would this be the dimmer switch , the light fixture itself or am I simply expecting too much from this economical led bulb ?
I don't want to assume; you say they were £19.99 for 5, but how many do you have installed? is it 5?, 10 or less?
dimming 5 GU10's is hit and miss at the best of times, but to be in with a decent chance, the go-to dimmer is the varilight V-pro (trailing edge dimmer).
Screwfix sell them, but I'm not sure if they're competitive or not. If your existing dimmer face plate matches other switches in the room and you want to keep it then you can just change the module and keep the plate.
I haven't tested those LAP GU10's from screw fix as they were never in stock; if you got them recently I suspect they are a newer model as when they go out of stock for a long time it usually means they're going to be replaced by something newer.