Thanks again guys. I went to my local bike shop and they informed me that my shrader valves were the cause. (Not suitable for high pressure, ie in my case 75 psi for touring tyres). So I bought some Presta valve tubes (recomended). Also got new BBB Windrush mini pump to pump upto 100 psi. Its actually smaller than my existing mini pump, so I hope I have made a wise purchase.
Tyres should now not deflate as before...I hope.
My stock of shrader valve tubes will now have to go in to store, till I next put on low pressure tyres.