4Granted
as I understand it, in order to unfreeze your frozen JAR ATPL you need a multi-pilot type on your license and you also need to meet the various hour requirements (multi-crew etc)
The HS125 qualifies as a multi-pilot aeroplane, according to the list on the UK CAA website (page 276 in the pdf doc)
http://www.caa.co.uk/docs/33/LASORS.PDF
How to get the type on your license, depends on the hours you have on type -
0 hours = full type rating course necessary (JAR approved)
<500 = reduced course
>500 excepts you from most training.
Either way, you need to complete the License Skill Test Multi Pilot Aeroplane (LST-MPA) and this must be observed by a CAA examiner. The LST counts as your ATPL checkride. LST format is prescribed, the check form is also available on the above website, it gives you a good idea of what to expect during the test.
If you haven't flown much around Europe, I would get yourself some sim sessions before the test, it is very easy to slip up and the test format is not very forgiving!
Good luck
South Coast where you hanging out.... any joy in orange land?