IMHO the diamond won't be a great starting point for doing an ir.Most people in the know seem to agree that the slower the a/c the better to give u more thinking time.I dont know the figures involved but im sure that the diamond is significantly faster than the seneca 1s that stapleford use!
Another point worth considering is the fact that instructors/examiners all know the piper fleet like the back of the hands and a fancy looking glass cockpit and totally different flying charcteristics will put even the experts on a steep learning curve for a while at least.
As i said just my humble opinion!
Ps Yes stapleford is a great place to learn!
PP