I would sell to the person making this offer. Whether you can ask for more or not depends partly on your negotiating skills, their level of interest, and their budget. Best-case scenario is that you'd get a little bit more in the low 4-figures. But it might blow up in your face -- meaning that, if you ask for too much, the buyer may become annoyed and walk away permanently. Offers on this particular domain won't be too frequent, although you could probably sell to other domain investors for low to mid 3-figures.
P.S. I'm thinking in dollars not pounds. Your offer is therefore better than I originally thought.