You can 'opt in' to the guaranteed 3p rate or 'opt out' and take any market rate you can get offered - which could be more or less than 3p. You can switch from one to the other once a year.
The guaranteed 3p rate is index linked to annual RPI change in the same way as the main FiTs rate.
I'd be interested to hear what views folks have on the likely rate people will get paid if they opt out. My gut feeling is that as energy prices increase it is likely to be better rather than worse than the opt-in index-linked 3p but other views would be welcome. We're on supply with Southern Electric and they will fortunately swap our meter for an import-export meter for free :-) Their opt-out rate is currently 3p also.
I'm about to register a 3.33kWp system and even in the first week of its operation it's becoming very clear to me that we'll export a very large percentage of what we generate in the week as we're both out at work all day. Of course the 41.3p generation payments are the bulk of the economic viability so opt-in vs. opt-out is not a huge deal but I'd still be interested in people's views.
Thanks,
Tony