If trained am I obliged to render assistance?
There is no Samaritan Act in the UK. That is to say that there is absolutely no obligation an ANYONE (including doctors and nurses) to render assistance to a casualty who has collapsed in the high street. You are perfectly entitled to walk by; you cannot be arrested for failing to act.
If I start to help am I then obliged to continue until the arrival of an ambulance?
NO. You may at any time cease your actions; you have no formal or legal Duty of Care to this individual. As an aside, if you are being paid to assist, for instance a First Aider at a music event or paramedic on duty, then there exists a D of C and other rules apply.
If, through my actions or inactions this casualty dies, can I be held legally liable?
Nobody has ever been prosecuted by the CPS in the UK as a result of the miss-application of First Aid, not least perhaps because there is no law on statute that refers to the topic. The golden rule here is that, providing you act within the realms of your training and the realms of your experience then you will be absolutely fine. That is to say do not attempt to insert pens into airways, needles into chest walls or any other procedure you may have witnessed on ER!
Act reasonably and within the realms of your training and experience.
For further information on this topic visit the UK Resus Councils website (http://www.resus.org.uk/pages/legal.htm)