They should be in Ericaceous Compost Mike, and fed every
6 weeks with Ericaceous Plant Food. I have also learned that they like a good soaking, if in a large pot.
Mine is a young one, has responded well, but the flowers are pink !
I have learned this year that this type of plant responds well if treated in the right way. My Blueberries are enormous ! No fruit this year, because I moved them from Limestone soil, hoping for a good crop in 2014.

Have you been pruning the hydrangeas Mike, if so you may be cutting of the flower buds.

I keep one hydrangea in a pot so that I can have a blue one (my soil is alkaline so hydrangeas in the garden are pink). I've found the one in the pot needs plenty of feed and lots of watering to get a good show of flowers. I also water it with Vitax Hydrangea Colourant which encourages the blue flowers. Agree with what Moon growe says about pruning.

Whilst I agree with what Mg and jaykaty say, I would like to suggest that instead of using vitax hydrangea colourant, you use aluminium sulphate to get blue flowers, I've had a look on line, and it appears to be about a quarter of the price of the vitax, and does exactly the same job, Derek.

If find the plants getting a little big or leggy, you can always prune 50% of the stems in alternate years, so you always have flowers.

Thanks everyone for this information. I realise now my Blue Hydrangea had been treated with one of these products when I bought it, although I planted it in a large pot in Ericaceous compost. It has turned pink now even with Ericaceous feed.