Well think I have discovered the source of the problem. While removing the NRV to fit a new NRV the base plate of the pump tube came away...
Thanks to all for the suggestions. Going to work my way through them while hoping it's not a crack in the pump tube. Thanks.
I found a paraffin stove while clearing out my Dad's garage. I think it is an Optimus 00 although it also has Primus parts. Replaced all the seals...
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