I looked up recipes online and eventually found one that did not involve using an oven. Filled the biggest pot I could find with chopped tomatoes, and everything was looking well....until I got bored of waiting for the sieved mixture to reduce and at the same time realised that my biggest pot was still not high enough to cover the jars for processing.
But I am an optimist and now have for jars of not quite paste, that need to be stored in the fridge, and should hopefully taste good and save us from buying two or three more jars of shop paste!