Sorry this is really embarrassing I feel bad now , it turned out that both dipped beam bulbs had blown at once, and somehow I was sold 2 new faulty bulbs.

So, Having tried all your ideas to no avail , I booked it in.

I’ve just paid £45 to an auto electrician to fit another 2 bulbs.

but your responses won’t be wasted , and there is a lot of good debate and info stored here for someone else in future. thanks all

Possible but implausible, sounds like an overvoltage situation but I am at a loss to think how that could come to be, hopefully the auto electrician will find the cause.