Is it cheaper to leave my heating on constant & just adjust the thermostat or have it on a timer?

Be interesting to hear other comments. I would suggest it depends on house occupancy ie occupied 24/7 or unoccupied during the day.
I take the view that if I go away for 2 weeks which would be cheaper, leaving the heating on for 2 weeks or turning it on when I come back.
I tried playing with this a while back but due to weather variables it was not conclusive so I (house occupied 24/7) have it come on low for during the day then around 6 pm it goes up to 19.5C then off at 22.30 for the night. I turn up when / if required.
If it was unoccupied during the day it would be a half hour blast for getting up in the morning then off all day and come on an hour before I come home.

Are we talking about a radiator thermostat on the boiler or a room thermostat?

If we’re talking about a thermostat on the boiler, you’ll save money by buying a room thermostat with a timer.

You’ll almost always save money with a timer thermostat though, but the issue is often how fast your house actually heats up when you tell it to get to temperature. If your house gets really cold overnight when the heating is off, and then you have the timer set to turn on at 06:00, it might not get to the temperature you want until 10:00… This depends a lot on the boiler, radiators and property though.

Without getting too data heavy, the general consensus is that many homes are too draughty to keep the hot air in.

Therefore, the best tactic is to have a timer to heat up your house just before you’re home/wake up. In line with the comment from @mowcius

You can also save huge amounts by setting the thermostat 1 degree lower.