Thank you both for the replies.
I kind of knew it but just needed it confirming.
I've turned my thermostat back up and the heating has come back on too, so I am very relieved.
I've booked a service with my plumber for January, just for him to look it all over.
Once again, thanks and...
Hi, utter newbie and total non-DIY-er having major panic attack here;
I came home from work and found that the house is too hot, so I turned the thermostat down a bit, went upstairs to turn the heating off a bit.
I realised that at some point I had flicked all the little teeth on the timer...