I requested a meter move from my supplier from inside the front door
to outside the front door. Following the survey we were told we would
need a new mains supply, which would require yet another survey and a
cost of £690- excludes the meter move (not yet quoted for !!) and the
electricians required.
I have tried to no avail to get the supplier to explain why we need a
new mains supply (and indeed why we are responsible for paying for
this)

Per the other reply, depending on where you're currently fed from may
have a bearing in this - you might be on a looped supply, or it may
not be practicable from your existing incomer for another reason.

If the existing cable is old, it is common to replace it (in ducting)
all the way back to the main - the existing incomer might be
undersized for instance, given the amount of electrical equipment in
use in a modern house. There are still houses in certain areas of the
country fed by a looped T&E lead cable and fused at 30A.

Cables can and do deteriorate with age, and may be replaced as part of
a refurbishment scheme, but in this case, you're asking for the work
to be done in advance of other refurbishment work - you get to pay for
that... Incidentally, even with a service refurb, the supply would
normally go back in the same place as the old one.

The meter should be on the same wall within 6 feet of the as the
incoming service cable / cutout.