Captain P
Well done on passing your PPL. In regard to your question, I would recommend against doing your IR now. My reasons for this would be;
1. The Single Pilot ME IR is probably the most demanding flying you will ever have to complete in your life. Ask anyone who has done it! Trying to convince your body it is good to 16 things at once is no mean feat!
2. Although I haven't done one myself, a PPL/IR on a single engine aircraft will be easier than its ME equivalent, so unless you want to pay to get a ME Class Rating as well I would avoid it.
3. If you want full information on the pre-requisites and the exam itself check LASORS. See Section E.
http://www.caa.co.uk/application.asp...detail&id=1591
4. Doing some instrument work prior to entering an ATPL course will obviously make the flying stage of the course easier for you. You might want to try an IMC rather than the full IR because it's substantially cheaper and more appropriate to flying singles.
Anyway, whatever you do, enjoy it and keep flying!
DM!