TOPIC:

An Ilco key blank KA14 X103 is a good match for the oem key in the pic above.
It's what I'm currently using. I got it from a hardware store & had them cut it.
The Ilco catalogue was kinda handy. their site has pdf downloads.

Latest news is that I found the seat lock and it does indeed have a 5 digit code stamped into it. Cant read it myself,Its too small,so Im waiting for number one son to get home and read it for me.
Apart from that,I discovered that the key itself has a slight difference in the teeth pattern on either side. It will only work in the seat lock one way round.I dont know if this is intentional or just a result of uneven wear.
So im still unsure if its the key that is the problem or the cylinder in the ignition, which is equaly impossible to turn,whichever way round I fit the key.Ill keep messing with it.

The reason,I discovered this now,is that Im piecing the loom together for a bench test,so wanted to turn on the ignition switch. A friend at work pointed out that as I had pulled the elecrical part off the bottom of the housing,I could switch the ignition on with my fingernail! Doh again! So,I can go on with my electrical testing and sort out the key when i get the time.

So,I couldnt get it working. In the end I took everything to a local locksmith. He said the cylinder was fine and the key ws the problem.He ordered a couple of replacements and recut them for me.Everything works fine now.
The new keys are a lot thicker and more robust than the old one,so should last a while and be less likley to snap off in the ignition.
Anyway,thanks to everyone for the replies and advice. Now you know,if anyones in a similar situation,just take the ignition to a good locksmith.