Most do. You can use Growmore, Bio, etc. Most of them have a balance of Nitrogen Potassium and Phosphorus

Thank you. I have been just using an all round fertilizer and the blooms have gone crazy throughout the winter and into Spring whilst in the greenhouse. Perhaps I will try Bio and see if this works better.

I wouldn't worry too much though. Mine are the same and still flowering brilliantly. They are a joy in the winter aren't they? Tough little things they are... If you overfeed you might get leaves at the expense of flowers!

Thank you for coming back with this:-)
Actually it wouldn't matter too much if there were less flowers because the blooms are so abundant, it wouldn't be noticed if they were reduced a little.
Yesterday, I thought it would be a good idea to take them outside, 10 pots ! The weather was mild and the forecast was OK so left them on the patio table overnight. I was upset to find that there had been a deluge of rain overnight which had beaten down the many blooms, and because the stems had grown long, they couldn't cope with the weight I couldn't have chosen a worse time to try to harden them off !!!! They are now sitting under a garden parasol trying to recover.

Oh how very frustrating - isn't that just the way of things?

Yes indeed :-)

Whilst on this subject, to another fellow geranium lover, can you please tell me why, the flower stems grow so long in the winter months, I wouldn't have thought they would be straining for light in a greenhouse?