Why not look here:
http://www.caa.co.uk/docs/175/Lasors_Section_F.pdf
or Subpart F of here:
http://www.jaa.nl/section1/jars/42/20/422078/422078.pdf
rather than hypothesising.
Differences training on an SEP is valid indefinitely and is covered by pre-99 grandfather rights. Differences training on an MEP is valid for 2 years but is extended automatically by staying in flying practice on the 'difference'. ME differences cascade down to SEPs (so if you're a current 737 pilot you get pressurisation and retractable gear for your SEP).