You pay no tax or national insurance on your stipend. You do not have to pay national insurance contributions to recieve free medical treatment on the NHS, but it may affect your state pension entitlement.
Actually, I'm not sure of the exact position on National insurance contributions re. pension entitlements. Any financial advisors out there?
If you're really keen, budget to pay into a private pension fund to make up for not having a pension from an employer for 3 years. This is rather good as even though you don't pay tax on your stipend, pension contributions still qualify for tax relief. E.g. if you contribute £78 a month to a pension, the government top it up by £22 to £100...more free money!