Yvonne, I guess you're in the UK as you quoted the cost of your cage in £s not in dollars. I have read several UK books and leaflets stating confidently that piggies either shouldn't eat lettuce or should have it only very occasionally (because it makes them sleepy was the justification). I have found that most of the books I read contain a lot of bad information and the advice given by experienced members and mods on this site is so much more useful.
I worked out that instead of spending money buying an inaccurate book on
cavy care, it was a much better use of my money to donate to this site instead and help disseminate good information as widely as possible.
I hope your piggies enjoy their (non-iceberg) lettuce!