Let me add a slight variation on the above, based on my experience this year.
I had 12 and a bit hours from 2004/2005 from when I started learning.
I then had an 11 year break, before deciding to just get on with it this year and get the licence.
I'd pretty much finished the syllabus by 50 hours (the 12 hours from a decade ago, plus 38 since April) but I'd read about the need for this exemption on a couple of forums (including this one) - so I sent off for it, and paid my £53 a couple of weeks before the test.
The exemption was granted and I took (and passed) the skills test at the end of August, passing with 52 hours (including the 12 from all those years ago).
The exemption is valid for 2 months and takes 10 working days to be processed, so only apply when you're ready for the test.