The same happened to me regulary, but i got it sorted, easily.
if you put your hand down the back and then under the engine, from the passenger side, you will find a load of leads(not to your distributer) even lower.
these leads are known to rust/chorode (info from an AA engineer)
This stops your car starting. Sorry i can't give you more details, but as you can guess i'm no engine expert.
but what i found is that every time my car died on me, i just reached down and moved the cables, then hey presto, car started.
Do Not buy a new battery!!! theres probably no need!