How to fix and overheat fault on my Glow worm 15hxi {3yrs old}?

on sunday i had a fault 4 {ignition fault- lock out when lit}, then in the same afternoon it went to a fault 1 {Ignition fault-lockout-boiler failed to light} It still worked a little but i had to switch the bioler off for around 2 hours just to build enough pressure up to relight it for approx 40 mins. Woke up frozen on Monday Morning with a fault 5 {Overheat fault} an it hasnot come on since then. When i switch it on it trys to light but wont and then switches itself off. Someone told me this happend to theres {just the overheat fault} and all they had to do was fill up a tap on their system, this cost them £80! Please help??