Oooh there is so much wrong with this I don't know where to start. You can't hybrid licences to meet your requirements.
My question comes in a number of parts; firstly, as far as I am aware I can 'piggy-back' my IR and MEP onto my FAA PPL which I can re-validate. Is this correct?
Nope, you hold a Full FAA PPL I assume from your opening statement with SEP. It is still valid and will always be valid, you just need a BFR. Once you hold a Full FAA certificate a 61.75 certificate is no longer open to you and you can't piggy back a non FAA IR onto a full certificate.
If your PPL is a 61.75 certificate then you can take the IFP test and add the ICAO IR as a piggyback, the same with the MEP.
I'm sure I am correct in that I cannot get a FAA CPL piggy-backed on my JAA one can I?
Yes, quite correct, you can't.
This is the concern I have! I work for company 'X' as a pilot flying a CJ for a private owner ('x' is the operator of the private A/C), and THAT owner also owns a light twin that he flies but wants me to fly on occasion for the sake or positioning and/or pleasure flights.
As long as the flight is a private flight then you can fly the N reg aircraft on your JAA licence. Outside the UK becomes a very interesting debate and one that is being done to death on the forums at the moment.
Secondly, is the FAA PPL (with the MEP-IR) sufficient for me to work for a company as a Pilot and to be asked by the private owner of the A/C to fly it for him? This is the real key to my question...
See my comment on licensing. The answer is no.