Thanks everyone for taking the time to reply - it's been really helpful to read through so many opinions
I deliberately didn't mention my instructor in the first post as I wanted to see if I was just making a mountain out of a molehill or if I was actually taking an unnecessarily long time in the circuit. What my instructor generally says is that it's not an unusual amount of time to take to get to solo, that I'm nearly there but that there's just a few things need tweaking, and that maybe in the next couple of lessons I can go up on my own. Except that he's been suggesting that I'm nearly ready for solo since around the start of the year... He's happy that the rest of my circuit is fine but isn't quite happy with my landings yet.
Much as I like my instructor and get on well with him, I am starting to wonder whether he's just a bit too laid back for me to make proper progress. What someone said above about 'keep on trying it will come right one day' does sound a bit familiar. He's also pretty young, and although I don't think for a minute that necessarily makes someone a bad instructor, I wonder whether it's partly down to having less experience that is making him more cautious about sending someone solo too early.
I know that having the big gaps between lessons is going to result a bit in having to spend the first part of each lesson remembering what I'm doing. To that end, my routine on a flying day is generally to sit down with my books for at least half an hour beforehand and review my notes from the previous lesson and the section in the Flying Training book that I'm on, and to mentally talk myself through a circuit and what happens at each point. I generally aim to get to the airfield about half an hour before my lesson, and have 15/20 minutes parked up on the road at the end of the runway with my airband receiver on to get my head back into the radio as well.
I'm going up with a different instructor on the next two weekends as mine is away, so I'm going to explain the situation to him and see what he thinks. I'm also in a position where I'm going to be moving house in the next six months and therefore also moving airfield. I'm suspecting that's going to involve a change in aircraft, so I think if I still don't feel like I'm getting anywhere over the next couple of lessons I might just take a break and have a fresh start somewhere else later in the year.