Hi Rosie
Have a look at mister-auto.co.uk
You don't say which type of silencer or what size engine you have, but you'll be able to get what you need on the above site.
If it's an underwing silencer for the small (850) engines, then Bosal 200-627 (£28.70) might be what you need; if ditto for an 1100 engine, then Walker 01538 is the one (£24.05).
If the silencer is at the rear end of the van, have a look at Walker 01677 (£20.38).
You'll know which one it is by comparing the pictures with the silencer you have on your van - alternatively, do a search using your registration number (bearing in mind, however, that the system currently on your van may not be the one originally fitted.)
They're a good, reliable company.
Good luck!
Andy