I have recently signed up to talktalk on the 40gb package. It is in a holiday home so the cap isnt a problem as the broadband is only used at the weekends. Is it possible that in the summer months when the internet will be used alot more that I could sign up to the unlimited package for 2 months and then downgrade again when they are up or am I tied into it for a year once upgrading?
Yes you could do, but why would you want to? There's no need.
TT's FUP allows you to go over the 40GB twice in any 6-month rolling period without charge, so effectively, you could fill yer boots for those 2 months without worrying as long as you know you won't go over again for a while.
Even if you did go over 3 times in 6 months, all that will happen is the download boost (nominally raising you to 80GB limit but actually unlimited) would be added to your account at £5/month and you can remove this after 30 days.
Hope that helps a little.