This is a strong and brandable name, I would probably extol its virtues and go back to them with an ask of £695. Personally I would be looking to sell above £450. I don't think it will be that easy to sell the domain above £600 in this particular sector, as I don't feel the domain is that essential to gaining new business, so I'd be fine with £450 or above. I'm sure these people charge enough for their time, so don't be afraid to ask for more if you want to ...
Just my own view, hope it helps.
Rgds