FlyingForFun is quite correct, if you already hold a CPL you are entitled to a 5 hour reduction from the full IR course of 55 hours (therefore the course becomes 50 hours
) In those 50 hours a minimum of 15 hours must be completed on the multi engine aircraft. A maximum of 20 hours can be completed in a FNPT G sim or up to 40 hours in a FNPT 2 sim.
In all honesty I think even the best students find they want more time on the aircraft than just the min. 15 hours.
As for taking the IR prior to the CPL, my personal view (having done the CPL then IR) is that this would be throwing yourself in at the deep end. The learning curve through a professional training course is quite steep and I found that moving onto the twin having been flying an arrow was a very steep curve!
The CPL course is little more than a regimented PPL with tighter limits and a few extra exercises, this is a logical progression from PPL to CPL. The IR then builds on your knowledge and experience gained in the CPL and adds another engine (amongst other things!)
The other factor is weather. If you are looking at starting your training in the UK in January then it may be better to start the IR first as the training is not as dependent on the availability of VMC, thus you can get airborne and complete an IR sortie in conditions that wouldn't be suitable for CPL training.
The CPL course requires a minimum of 5 hours in the complex single aircraft. Some students seem to prefer increasing this to around 10 hours of the course, as they do not feel confident going for test with this little experience on a complex type. By completeing the IR first they would have gained the experience and feel more confident in the CPL syllabus.
I hope these points are of use to you, if you need any further information i am happy to help.