Blown/Melted Fuse

Hi there, my Electramate 2000 has only been producing luke warm water for the last couple of days. However over the week I've noticed a chemical/plastic smell coming from the cupboard. Well I took the cover off the boiler to to see if there was anything noticeably wrong with the boiler due to the lack of warm water and saw the left hand side 30A relay wire coming out from the top of the breaker, completely melted and the small switch/box/fuse above the breaker is melted partially too. The forum won't let me post pictures yet so its difficult to describe.  
I've booked an engineer from Gledhill but they can't come for 10 days. I'm worried a) what caused this and b) will this will get worse i.e. catch on fire (although it looks like the worst has happened), what do you guys think? Is this easily repairable? How much do the parts cost? Thanks in advance.

Sound like relay trouble.
Or wire was not tight and burnt out.
Mike