I'd recommend you sit down and read LASORS at some point. One would expect you to know the answers to these questions.
Unless your license has expired (5 years) or SEP rating has expired (2 years) there is nothing to revalidate on your license. Get in and fly... although an instructor checkout might be wise and will almost certainly be mandatory if you're renting.
As mentioned, all exams must be completed within an 18 month period. If you started last summer, you've got until Christmas this year - make sure you leave time for retakes, should you need them.
I've always wondered... Say you sat your first lot of exams in (say) Jan 2009, then did the second lot in May 2010 and failed one taking you over the 18 months. The CAA would try to say you have to start from scratch, but what's to stop you saying "Ah no... I decided in Feb 2009 to jack it all in and start them all again from scratch, therefore the May 2010 exams signal the start of my 18 month period" thereby retaining the credit for one session. Anyone bumped into such a scenario?