OK last tax year (2006-7) we were over paid quite substantially on our TC (our fault). This year they've cut out payments dramatically (from about £300 a month down to £27) - some of that is "repayment" the rest of the cut is down to the fact that our joint income was pretty big last tax year.
Now AFAIK if we stayed together the Tax Credits next year would still be pretty low (possibly only because of high salaries but not certain).
Anyhow, IF it turns out that next Tax year our payments are still being reduced due to previos over payments how would that affect me claiming TC on my own with a greatly reduced income???
ie - when H moves out (before the end of the tax year) and I tell them the changes will the payments for this tax year (and next) then be based on my income for the year (a "massive" £4500) or our joint income (nearer £30k?). And what about next tax year when it will ONLY be my income that is used???