Hi, Jowie -
Since it's a G4 (AGP) model, that opens things up quite a bit.
You can use any retail (white label) OS 9 Install CD with that model - OS 9.0, 9.0.4, 9.1, or 9.2.1. You can then use download updaters to bring it up to the version you need -
Article #60791 - Mac OS 9.0.4 - Download and Info
Article #75103 - Mac OS 9.1 - Download and Info
Article #120030 - Mac OS 9.2.1 - Download and Info
Article #75186 - Mac OS 9.2.2 - Download and Info
Note that the preceding updaters must be installed in sequence. For example, if you use a retail OS 9.0.4 Install CD and want to end up at OS 9.2.2, you would need to apply the updates for OS 9.1, 9.2.1, and 9.2.2.
I referred to retail OS 9 Install CDs rather than original Software Install or Software Restore CDs because the original disks are extremely difficult to find. A Restore CD also is not easy to use - it is not difficult to accidentally erase the whole drive when using one of those. Also, it is very hard to get just the OS 9 install from one of the Restore CDs - they are designed to revert the drive to the original as-shipped configuration, including any additional software originally bundled with the machine.
So, where to get retail OS 9 Install CDs....
When Apple stops selling a particular OS version (usually because that version has been replaced by a later version) they apparently sell off their stock of retail disks to 3rd-party vendors. These vendors still make them available. In the U.S., other than LEMSwap there are sources like this one -
http://hardcoremac.stores.yahoo.net/
I don't know whether that source, or others like it, will accept a purchase from buyers located in other than the U.S. - some sellers will, some won't.
You can also check with an Apple User Group for their suggestions - most big cities and universities have one. You can search for one near you here -
http://www.apple.com/usergroups/
EBay is another possibility, but be cautious - sometimes a seller there will describe a model-specific (gray or gray-blue label) disk as being suitable for any model MAC, when that is not true.
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Another item to consider, perhaps. You're located in the U.K., so might need, or want, to acquire the International English version of the OS rather than the North American English version. Unlike OSX (whose install disks contain all available languages), OS 9 comes in one language version per disk - and there is no way to convert an installed OS 9 to a different language.
I'm not sure what all the differences are between the International Ennglish and the North American English versions, except that the download updater(s) used must match the language version of the installed OS 9.
One difference is in the keyboard layout - the Int'l English layout is a bit different that the NA English one, placing certain characters (such as the symbol for monetary pound) in more convenient locations. There is a choice of keyboard layouts in OS 9 - for example, a machine running NA English can be set to use the keyboard layout for Int'l English. Whether this would be sufficient, I don't know. Another reason to consult an Apple User Group, I imagine.