Andy,
I had the same problem on my neighbours boat last year - same charger and same fault. Upon inspection it was the input transformer that had failed on the unit. I spoke to Mastervolt and they admitted there was a component fault on this and some other models which was a common point of failure - check out my previous posts.....
Speak to the Mastervolt sevice agent in the UK advise them of the serial number and tell them the fault and that you know there is an issue with a component in these models. They will charge you a flat rate repair for the charger circa £150 quid....they did fix some units free of charge but depends on how old it is but you will probably have to pay the fixed charge to get it fixed.
There is no quick fix - unit has to be removed and returned to Mastervolt.
MASTERVOLT UK, LTD. (AREA HEADQUARTERS)
Address & Postal code: Winchester Hill Business Park, SO51 7UT,
City: Romsey, Hampshire
Country: United Kingdom
Contact: Mr. Ian Kitchingham
Phone number: +44 1794 516443
Fax: +44 1794 516453
E-mail: info@mastervolt.co.uk
Website: http://www.mastervolt.co.uk