I overlooked the fact that you have a TR. Under certain conditions, you are allowed to take skills test (i.e. full two day sim) in a sim (same type, obviously) and gain your JAR multi-IR. Not sure how that would compare financially to going to a flight school and doing it in a Seneca (15 hours plus test).
The CAA are pretty good at giving licensing advice if you email to them. Takes a few days, but you usually get a clear response. Well worth doing before you start spending money!
Regarding your question about an airline hiring you and getting your IR during the TR, it's pretty unlikely especially in this climate. A frozen ATPL (i.e. JAR multi CPL/IR and ATPL written) is the strict minimum.
Not the end of the world if you don't pass IR first time, but basically if an employer has two similar CVs and one has first time pass and the other not, it might make the difference.
P