The 48k model only had 32k of available ram, of which roughly 8k was screen memory and system variables. The lower 16k was ROM.

Nope, the lowest 16k of address space was ROM all right but on top of that the ZX 48k had a total of 48k RAM (as the model name implies.)

The lower 16k RAM was in fact a bit special in that it was shared with the graphics chip (which read the lowest 8192 bytes of RAM constituting screen memory and interrupted the CPU while doing that) System variables (POKE for the win) sat on top of that. The upper 32k RAM was a bit faster since CPU had it all for itself.

Sorry you're absolutely right. I'm getting mixed up with the use of 64k chips in the 48k model[1], which was a cost cutting exercise allowing sinclair to use potentially faulty 64k RAM in the 48ks.

[1] - http://www.users.globalnet.co.uk/~jg27paw4/yr07/yr07_33.htm

Maybe it was 5 minutes, but at age 9 everything seems bigger (and take longer!).