Nigel,
At my place of work, the vast majority of our computers used 8 inch floppies. These were 360Kb (single sided single density), 720Kb (double sided single density) or 1.4Mb (double sided double density). Actually, I've a vague memory that last one was known by its unformatted size, but am open to correction.
Apart from using them for data storage, they also had the CP/M operating system on them.
I don't recall seeing many 5 inch floppies until one of my colleagues managed to get hold of an early IBM XT which came with two 5 inch drives. This was in the late '80's.
In general, Clive Sinclair was quite good for me in that work bought me a Mk14 kit to learn on. I later bought a ZX80, then the modification to make it a pseudo ZX81, followed by a Spectrum 16K, Mk 2 version I believe. The Spectrum had the "rocky" 48K RAM pack added to it, and later a Rotronics Wafadrive along with Sinclair's thermal printer. Ultimately I built the whole lot into a homemade box including a ZX81 keyboard (modified by changing the labels to suit the Spectrum) and that lasted until I replaced it by a Sinclair PC200, an IBM XT clone.
Using the Sinclair machines enabled me to understand how these machines worked, and introduced me to both BASIC and machine code. Thankyou Clive.
Incidently, I believe that Spectrum is still working: my elder son has it, and last I heard he had got it working again. Mind you, that's a few years ago now.
Peter G. Shaw