Thanks for all replies and info, I'm now closely checking everything as it's to easy to blame one thing for another. I did another brew with just treating the water with ph stabiliser which again had a ph5.8 as always taken at room temperature from a sample 10 min into mash, this brew came up...

I've just managed to completely pickle my head! In previous brews of light blonde ales, I've aimed at and succeeded to treat my water to get a target pH of 5.2 to 5.4, this is measured from a sample taken about 15 min into the mash, but my target OG has always fallen sort by about 5, ie 1.033...