Glyn, 14th july is the public holiday, in N. ireland only. The organisation you work for determines whether overtime etc is payable - eg some places have a 'buy-out' agreement that gives workers a little extra hourly pay in return for giving up their entitlement to double time etc. So first of all check what your terms and conditions are - your employer cannot refuse to give you this information. This will make it clear when and how you can claim overtime. Similarly it should clarify how you are called on to work - is it a shift pattern, is it regular mon-fri, or what.
Lastly, I know I say it every time, but join a union and then your rep will do this kind of asking for you.