They could well have looked at it and there wasn't a leak, you could drive a mile, check again and there's a leak. So they are correct in saying it's nothing to do with them at that time.
However, if the part has failed (aka a leak is now present) and there is a recall to check for that very thing i would expect it to be covered, that is the whole reason of the recall, had the inspection found something they would have had to do it at the time. Since subaru is aware of the problem, i would expect they can claim back the cost from subaru anyway.
I would speak to the DVSA and ask the question. I would expect they will take a very dim view if you are being fobbed off. !!!!!!!!!!!!!! It's the DVSA that deal with recalls, not the DVLA!!!!!!!!!!!!!!!!!!!
Other option is speak to Keith Price subaru south of you (its 3 miles further way so not a major difference) and see what they say, the subaru dealer network is very hit and miss, i have one within 2 miles of my house (geoff cox) and they made a complete hash of some work on my mrs car, yet europa up north have been great.