Hi, welcome to GoY, I would just use a general purpose fertilizer, such as growmore, Derek.

I agree with Derek. Growmore, which is a slow release fertilizer will do the job. You could also take the organic route and use well rotted manure and mulch. This will have an added effect in that it will keep moisture in the soil as Hydrangeas are thirsty plants.

Nothing at this time of year. Late winter/spring is the time to fertilize hydrangea. In the current spell of dry weather they will benefit from a mulch to keep moisture in the ground.