*nodnod* London clay is pretty alkaline, I've found, and widespread across the south east. It's possible you're on that. My hydrangea is pink, I'd rather it was blue but it's too big to dig it up and put it in a pot as it's been here longer than us LOL. You could also top dress with some ericaceous compost - it's sold at DIY places and garden centres for heathers, rhododendrons etc which like acidic conditions.