OK I closed off both valves (the one near the floor and the one on the RH vertical pipe) and the intermittent pumping stops. Turn them back on and after a minute or so the pumping starts again so I guess we're into leak territory.
Do you know anyone in the Hua Hin area who has the equipment to locate where the leak may be occurring (if its under ground)?
I'll go round the house again (inside and out) and check faucets, toilets, washing machine?? but if there is nothing obvious then I might need someone with the right equipment to detect where the problem might be.
Thanks for all your help BTW and this is conveyed to all the respondents since I posted yesterday....I really appreciate all your help.
Thanks.