Hi Madgirl
Ignore everything you have heard or read already. I profess to be something of a pensions expert.
As a student you are neither contracted in or out of S2P- you need to be paying NI to do so. Therefore you do not have an option. If you were working it may be better to contract in, the reasons why are way too complicated so I won't discuss that further!
However, once you join the NHS scheme you will again be contracted out (the scheme is contracted out so you have no choice). Unlike before however, your NI rebates are not going to be paid to your personal pension but the NHS will provide a level of guarantee d income to replace the fact you will not receive a state second pension. The good thing for you is you will pay a reduced level of NI!
As for your existing pension arrangements, it might be best to transfer your final salary pension to the NHS scheme once you have joined (unless your existing scheme is a public sector scheme, the NHS scheme is likely to be better). Alternatively you could transfer it to your personal pension (which given you will be a member of the NHS scheme probably wouldn't be best advice) You would need to obtain sutiable advice before doing so and this willl cost you unfortunately (the advisor must have the G60 exam in order to advise you).
I would imagine your best bet would be to transfer the exsitng final salary scheme to the NHS scheme- whoever you choose to advise you on this will charge you a fee for doing so- be very suspicious if they advise you to transfer it to a personal pension!!!
A for your existing stakeholder pension, changes from April 2006 mean you will be able to take 25% of this as tax free cash if you require it, whether it came from SERPS or S2P.
I hope that all helps.