Autofly,
I am in the hour building phase also. The best thing to do is to go on to the CAA's web-site and get some details on the CPL course, and then base your hour building on that. For example, aim for CPL handling tolerances as you have mentioned, practice PFLs, steep turns.... Obviously you wont be able to practice IMC flying by yourself. Also, have a look for a thread in this forum called 'CPL diary' by flyingForFun. The thread is very informative and interesting.
In your hour building phase, you will have to accumulate an amount of dual time, that depends on how much you currently have. If you have more dual time to build, than dual time that is required for the IMC course then go for it. You need to build the dual hours up anyway, so getting an IMC rating at the same time may me be a good idea.
In reality though, and this is what I have found since I have been building my hours slowly, is that the 'dual time building' accumulates through checkouts, since the British bad weather has meant that you have not been able to fly for a while.
As for getting a twin rating now, in my opinion, don't do it. With the amount of hours that you have got, the money is better spent gaining experience in a SE aircraft.
Hope this helps,
PA28.