not sure it really answers your question but...
I claimed mine in the year that I did it, but my accountant said it was only claimable against income in that financial year, so it wasn't as lucrative as it may seem.
eg. if you paid £8700 for your rating, but say started work towards the end of the financail year only earned £4000 during the same financial year then you can only claim against that £4000 - so you wouldn't get much back.
It also depended on how the rating was paid for and what the contract said. (I'm not an accountant so I'm just remembering what my guy said) Basically If it was a 'speculative' type rating then definately not. If you had a letter and contract of employment that said 'you've got this job and have to pay £xxx for the rating' then it MAY be claimable.
You almost certainly cannot claim an 08/09 rating against 09/10 income. But wether you can retrospectively claim an 08/09 rating against your 08/09 income but putting it on your 09/10 return is a question for your accountant. Make sure you ask him the right question though! he may think you've asked wether you can claim last years costs against this years income.
On one final note, most tax returns are processed by some external agency (GE Captia or whomever the government employs this week) by a bunch of minions punching it into a computer. HMRC inspectors then audit SOME returns. If your return gets processed, you get a cheque, then the taxman only has 12 months to change their mind. If they don't audit yours or they do it outside 12 months then you keep the dosh. So 'grey area' stuff may be worth a punt anyway.