1. To obtain a SEP Class Rating, you will merely need to take the Renewal LST with a PPL Examiner, then send the application off to the CAA to have SEP Class Rating privileges added to your ATPL.
2. If you have a current service aeroplane IR, get an IMC Rating added to your ATPL as well. See
http://www.caa.co.uk/docs/175/SECTION%20E.pdf page 13. Your ATPL multi-pilot IR does not give you any instrument privileges on single pilot aircraft any more.
3. For QFI-to-FI accreditation, see
http://www.caa.co.uk/docs/175/SECTION%20H%20.pdf section H1.8 on page 8. If you're an A2 or A1, then the 'no aerobatics' and 'no applied IF' restrictions will not apply.
If you need somewhere to do the appropriate training,
http://www.ontrackaviation.com/ at Wellesbourne Mountford (near Stratford-Uponavon) are highly recommended. Mostly ex-CFS and very familiar with mates in your situation.