I ordered some cartridges from Tyre Inflators, which works out to around 75p each, including P&P.
I went to
Slane Cycles for the M-Part inflator which Simon gave me to use on Friday night. I liked the fact that it screwed onto the tyre, and has a knob to open and close the valve. Something with a lever triggered valve and a partially used cartridge is soon going to empty itself into my bag, or jersey pocket.
I would have bought some 25g cartridges (which I believe is what Simon gave me with the inflator on Friday), but they are more than twice the price of 16g ones (at least from Tyre Inflators), so I'll just use twice as many small cartridges instead!