I was vaguely aware that this might happen but I just ignored the niggle in my mind. As I have been staggering my seed sowing in the propagator some of them have sprouted before others. Namely, the peppers which were sown almost four weeks ago and two of the tomatoes sown two weeks ago. Interestingly one of the tomatoes is from the same batch only beginning to sprout now.
The problem is that the seeds I sowed last weekend have not sprouted yet whilst the other seedlings are getting to an inch tall and were touching the top of the propagator. I need to take the lid off to let the sprouted seedlings fill out and not get leggy, whilst keeping the un-sprouted seeds toasty and warm inside the propagator. The solution? I took the lid off and placed an up-turned vegetable container over the cells which still need the extra warmth. The fit is quite nice and it looks like this should work.