If its 280 based, it'll be fine with a UK spec spring , Dave. If it's a 240 under there it's not going to put out the required power regardless of spring.
I agree that the old medallion ploy would have been nice. The 35C is growing on me as it is, I must admit. A nice wee plinker with open sights (and not the Tru-Glos either), I'll wager. Im sure I'd be able to find a use for it at 7.5 joules!