If your XM has a prefix of BFA then it will (if not already done) need a valve replacement. It's free of charge but can only be done by an authorised dealer
I understand there was a recall within the last couple of years and I only found out by chance when I took the raft earlier this year to a local service station and they couldn't do it because of the recall.
Try Ocean Safety, who have offices all over, including Premium Liferafts in Burnham-On-Crouch. These guys are servicing ours at the moment although it's not an XM one. A quick phone call and you should know if they can handle it.