Thanks. Well maybe I just need to apply a bit more force. I'll give it a go.

But in case that doesn't work (and it seems I'm not the only one in this position) - surely the clamping of the pitlock is done at the point where they meet the dropout: at that point, they can't fix the wheel in place firmly enough. But if the tension keeping the wheel in place is provided by something else, then that shouldn't be a problem. That's why I suggested using a chain tensioner, which is also called a chain tug - I don't mean the spring-type; just something that's inserted into the dropout to keep the axle in place. (I'll try sending a link, but when I last tried I wasn't able to because I hadn't posted enough.)