(As an aside, the catalogue number OR76BOOM01 is brilliant. Top marks for whoever came up with that one)
Sadly, the gun is non-operational so there's no point in stocking up with ammunition with name of your nemesis on popular web forums inscribed on the outside.
For operational model warfair, you'll need to track down a Triang R216 Tactical Rocket launcher. Launched (geddit?) in 1957. Available with several different colour rockets and launch devices, it's a proper collectible, but available for surprisingly small amounts of money. This one only cost me a fiver a few weeks ago from Footplate Models in Kidderminster.
The rocket sails several feet when fired. I suspect that the spring has weakened over time and when new it would be a pretty potent weapon against your kid sister or other enemies.
I'm not one for war toys at all, but I can't help liking this brightly coloured accessory. Perhaps back in those days, people didn't take the hobby so seriously? Perhaps the "Boche Buster" will show that many still don't, they just want to have fun.