Thanks for all of the advice so far. In answer to the question about swapping the speakers - yes, I've tried swapping the connections so that the left channel on the amp is hooked up to the right hand speaker and vice-versa, and it makes no difference. The left is still louder.
I hadn't thought about having a hearing test, but I think what I'll do is get a second opinion on what I'm hearing, since it may be a while before I can get a test. I'm intrigued at the suggestion that the composition of the walls might have an effect. The left is close to a very solid exterior wall and the right placed at the same distance from a single brick interior wall. Quite how I fix that, apart from a lot of trial and error, I don't know, but it could be fun.
The technical information is intriguing but well beyond my limited abilities so I will resist any urge to open up the amp and start fiddling around. There's a very good hi-fi shop (from where I bought my current turntable and speakers) just down the road, so if the problem persists I will drop the amp in there when it re-opens. I'm sure they will try to sell me a new amp and, since the incumbent is 31 years old I might stand a chance at persuading Mrs Redlight that the purchase is justified