They could very well be loading your premium for that claim then. It would be interesting to see how much your renewal will be on your current vehicle
I'd almost guarantee that's the cause. Insurance companies are crafty - even with protected No Claims Discount, if you claim, your wallet still gets mugged at renewal because your premium goes up. For example, if you got 65% NCD and your premium was £400, you may find that you still get 65% NCD but your premium is now £700.
Now that Chris Knott has ties to the forum I'd give them a try first, otherwise try MoreThan, who, as you can see from my posting in this thread (scroll down a bit), viewtopic.php?f=34&t=6086&start=30
actually give a 10% DISCOUNT for LPG.