I can let u in on the price matey but i hope your sitting down! Due to the fact that the 20v engine is a hefty wedge to fit in a marea and the fact that the boys at fiat did'nt really think about general maintainace, the engine has to b removed ( although i have heard that it can b lowerd and tilted but i'm not sure ) so it's a good 8 hours work and the belt comes as a kit which is not cheap so be prepared to part with the best part of £1000 give or take 200 quid'but some garages may not touch them cos of there complexity.Sorry to bare the bad news
if the bill is very high say you are not paying extra for him not being very good at it, say you know lots of people on ff who have had it done for under £400