I have just returned from the local shops TRIUMPHANT!!! The butcher who, he says, is a vegetarian and an engineer, managed to fix the cooker. They said the stock smelt great but I will begin again as it is about 3 days old. lol. The knob on the lid had worked loose (well it came off in my hand) but how he got the glued on lid removed from the pot, I do not know.
He was joking about the vegetarian bit. One never knows when a butcher is being serious or when he is joking...... They reckon I had put too many chicken frames and wings into the cooker and had overloaded it.
Local butchers are very multi-skilled indeed.
Thanks for the offers of help, very much appreciated.