Question Author
kvalidir - "It's a complete scam, very well known, it might appear to clear after 3 or 4 days but in reality won't have and will be returned at a later date because banks are set up to clear after 3-5 days even when the cheque can still bounce. It's particularly dangerous if you take a cheque from abroad as it will show as cleared after 3-5 days but won't be and can bounce up to 6 weeks later."
Thanks for the additional information.
Prior to posting, it was always my intention to take my cheque into the bank and confirm its veracity, as well as confirming that the money had arrived in my account, before sending any money back.
On reflection - the notion of a total stranger sending over £200 to another total stranger on the basis that they will refund 75% of it drives a coach and horses through the believeability of the entire set-up.