One thing many seem stuck on is 'has to come from the manufacturer'- grab a set of calipers (or even a ruler will do usually) measure the shaft diameter (yours appears to be a keyway pulley so add that fact to the shaft size) and the outer pulley dimension and look for V belt pulleys with those specs...
That looks like a keyway (bottom in the center)
Pulleys are available in a HUGE variety of different sizes and types- the hardest bit will be finding one that is the same size as yours in amongst the thousands/tens of thousands of variants available!!!
Don't know about the UK, but I use Aims here in Australia- have found some pretty oddball stuff just by doing as I outlined above- and they are just one firm here that has a huge selection to choose from- remember most electric motors are (despite the manufacturers labels) usually only a limited number of shaft sizes, same with pulley diameters tend to go in similar 'steps' in size, and V belts are only made in a limited number of different 'V's sizes...
ETA- you might even have a couple of alternative mounting options there- there appears to be a thread on the actual motor shaft with a 'keyway adapter'- if that is what is there (bit hard to judge from that photo but definitely looks like it may be) then you could either go the keyed pulley or a screw pulley (or get a blank shaft pulley and thread it yourself with a tap...)
There's a LOT of options out there... (steel or aluminium or alloy- even plastic/nylon ones, single speed or multispeed 'stepped' pulleys)- if it's been made, it's likely available from companies that specialize in them... (and likely much cheaper than a OEM manufacturers 'spare part' - even assuming it is available still through them)
Just something to consider...
