Right had a spare half hour so decided to go through all the channels to see which was best. Channel 11, 12 and 13 were all great. Settled on channel 11 which now shows very good signal strength and 72mbps (occasionally drops to 65- might stay on this for a bit but goes back to 72- or 52 for a few seconds)
But.........................speed test on thinkbroadband is still showing 5mb or 4,4 mb sometimes. Only objects in the house that have changed are a printer that is sitting next to my pc on the floor next tot eh wireless adapter. And also a wardrobe that was moved from my 3rd floor room to the 2nd floor. Very odd problem!