Thank-you all for your good advice!
I've bought my new mattress now.
I went shopping yesterday, visiting The Bed Factory in Bridge street first. While I liked the feel of 3 mattresses there, 2 were brands I'd never heard of, (both £119,) and the third, a "Sealy Orthopaedic", wasn't available in single size.
I took notes however, then went up to Bromborough. The place that I remembered has moved to much smaller premises, but I still found a mattress I liked, a "Bedmaster Luxury Orthopaedic", £130 with delivery.
And it was delivered around 3:30pm today! How's that for quick service?!
Ahh, I will sleep like a Baby