Hydrangea on Chalk/Clay

We have a hydrangea in our back garden, it came with the house. I've blipped it before but it's here again because I'm running of things to blip today.

Our soil is clay with flints (lots) on chalk so very alkaline and gives you red/pink hydrangea flowers. We'd need to dig in lots of acidic/organic material to make them go blue. Only when the pH is low enough can the absorb the right minerals to make blue flowers. In Brittany where my better half hails from you get amazing displays of these flowers.