Moral issues aside for a moment, I’m afraid there’s not much you can do.
The legal process is weighted in favour of the Consumer / Individual against the Business, because usually it’s Mr. or Mrs. Joe Bloggs against Global Conglomerate Plc.
You can’t turn up where they work to even ask them nicely about it, that could be seen as harassment.
They don’t ‘serve’ people, they send them letters saying pay or else – the else is a court case which may end in a CCJ – when people are in the habit of doing a bunk their credit report is usually a mess anyway and it won’t phase them – you can take it to the high court who can enforce payment (see – can’t pay, we’ll take it away for a one-sided view of it).
I suspect your no win, no fee solicitors won’t want a high court case over £800 because even if they are awarded fees, they might not get paid.
Personally, as someone who spent a few years trying to force people to pay what they owe, with the backing of a large legal team, a budget and completely water-tight paperwork and paper trail proving they owe the money – I wouldn’t bother for £800, some people are arseholes.