I have mattress toppers on all my beds. They are great. Good quality fitted sheets should be ample enough to cover them and the mattress, but sometimes the cheapie sheets are a bit skimpy on material. You can buy extra depth fitted sheets though if you need them. Mattress toppers are great. I LOVE mine. I don't want a full mattress of them (too hot in the summer) but the toppers are great. Get a good quality one though. It's false economy to get a cheap one. Sleep well Sue xx
Depends on the depth of the topper you buy. Thinner ones won't notice, thicker ones, well worst comes to worst you can put the sheet over the topper not the mattress. I suggest get one and find out :-)
Here in the U.S., mattress "pads" (as we call them) are designed to fit anything up to 18" deep since all the corners are elastic.
If they're available in the U.K., I highly recommend the type of 'topper' that is designed to be cooler, since our "memory foam" pad (about 2.5" thick) tends to be somewhat warm, especially in the summer. The cooler version makes a noticeable difference. They can be bought to fit pillows as well...