My nephew has given me his Xbox 360 to see if I can get it working again, three red lights, I figure its the old heatsink problem, but I can't get the sodding white plastic cover off, any experience with doing this?

No but you can pick up a removal tool off eBay for about £1. I bought one for my dad (who was repairing my nephews console) but he got it off in the end without it.

Well I managed to get the case off by breaking it, stupid thing, anyway, after applying Arctic Silver 5, re-fixing the heatsinks and deliberately over-heating the thing, I've got it to display again... here's hoping it's not just a temporary reprieve.