Hello albwown!
I collect the sales prices for these guns in Germany at an auction site called "egun".
You can download the data at this link. (XL-file, updated every week at the same link)
I don't know how valid it is in the UK, but it should give you a pretty good idea since there are almost 1700 FWB300S prices. Generally the price is about 160-170 Euro, 20 Euros higher for the Universal-model, and about 20 Euros lower for the FWB300 (without S), which is an older model.
Hope it helps...
/ Micke