AFter a great deal of googling and reading I discovered it is possible to get bots working with Steam.
The key is a thing called metamod. This allows the use of admin type plugins in Steam, including bots.
It's a bit fiddly to get working. Actually let me clarify that... I had no luck whatsoever getting it to work with halflife deathmatch.
The idea is you install the metamod dll into a specific folder structure within the mod that you wish it to work with, then you edit the liblist.gam file for said mod, so for the game_dll it points to metamod, which in turn will run the actual game dll. However, for halflife death match there was no liblist.gam file.
I tried editing a copy of the one in the counterstrike folder but had no luck getting it to do anything. So I gave up with it.
Instead I turned my attention to Counterstrike, as really that's rather closer to the ultimate goal of playing insurgency anyway.
After a bit of messing about (read that as me naming a folder wrongly so counterstrike couldn't find the folder I pointed at in the liblist.gam file...doh) I got this working.
Then I instaled RealBot, which was simply a case of unzipping it into the correct folder structure (which was nicely explained in the readme), copying a command.txt file into the counterstrike folder and creating a file in the metamod folders which pointed to the Realbot dll.
Once in game there is an easy to use menu which is brought up at the press of a button which allows you to easily add/remove bots to/from either team.
Ofcourse as ever with computers, by the time I'd got it working, it was a bit late to actually try it so this evening my girlfriend and I are going to be taking on hordes of terrorist bots!!!
Valve