One room is always cold

FreeBear

ThisIsWeird said: When you move, check the insulation levels of your new place closely! Or, factor in having it improved at an early stage. It is disruptive, yes, but actually quite easy, to insulate a room well.

That said, if there are sockets on the external wall(s) and a radiator mounted under a window, the difficulty and costs rises. Slapping a bit of insulated plasterboard on a wall isn't very difficult, and if you are doing the work yourself, not hugely expensive. But if you are paying someone to do it along with moving radiators & electrics, it can get expensive. The finishing details can also bump up costs...

Oh, and when it comes to fixing stuff to the walls (radiators, curtain rails, wall cabinets), things get <um> interesting.

I know when I rip skirting boards off, they will not be in a fit state to put back on (currently fixed with multiple cut nails, and the boards will split). So I either need to replace all the skirting or try and find a short length with a matching profile. For the former, I'd need to spend ~£400, and for the latter, it just isn't going to happen, so would need custom made lengths (been quoted ~£800).

Aliss08

If the radiator is on an external wall I would really recommend trying this.  It made a big difference in my North East facing bedroom.

pieroabcd

Is the radiator in the main bedroom cold? If so  you may have ab unbalanced system, as someone else wrote, that means that the radiators at the lower floor steal all the hot water from the ones at the 1st floor (and higher).
You can rebalance it yourself. It's free and  generally very effective. In my case it literally made miracles, even though I have to say that my bedrooms are all west or east facing.

silverwhistle

I've a mid terrace in the south so don't have the issues you face even though my bedroom faces north and is cooler, which can be a relief in summer. But I never have my radiator on at all and rely on swift entries and exits to my warm bed and heat filtering up from below! So some insulation would be my first port of call and good ventilation, as pointed out. Can you borrow an infra-red monitor to check for where the worst issues are?

Bigphil1474

Our attic bedroom is always cold in the winter. I can second the rad foil stuff, made a difference of maybe 5oC in the attic for us. Unfortunately it's a dormer with little insulation so without spending a fortune it's difficult to do more. We don't use it much though. You haven't actually said whether the radiator is hot in that room? Our thermostat is in our main bedroom and that is usually closest to the thermostat temperature of all our rooms, as the lower floors tend to be warmer.  

ThisIsWeird

The OP, CCJ, did mention it was an all-year-round issue, so seems pretty clear it's a mix of house direction (North-facing room) and lack of insulation. It's obviously worse in winter time.

The foil idea is always worth considering, but I don't expect it to fix anything significant. It also runs the risk - if a small improvement is detected - of being tempted to run the CH rad in the bedroom for more extended periods of time. Again, this might make things more 'pleasant', but it'll be at a cost.

Of course, if the use of foil does help enough, so that the CH rad can be used instead of a leccy oil-filled, then that will be cheaper - but only if it's 'timed' to come on when needed. That will need lots of running up and down the stairs to turn the rad on when needed.

The MSE way is to heat a room only when required.

CreditCardJunkie

Thank you all for your replies, you have no idea how much I appreciate you all taking the time to advise! We are going to try both the foil and oil filled radiator with timer & leaving the window vents open 🤞

Just to clarify the radiator does get very hot but doesn't seem to heat the room enough. We do always leave the door fully open as the little one tends to wake in the night and make her way into our bed.

ThisIsWeird

That's understandable, but might change matters - I'm not sure. If you leave the window on vent overnight, then it'll likely create a steady draught through the whole house - all your lovely heat will be vented away, to some degree, at least.
My son's room - although south-facing - is easily the coldest bedroom, presumably because it has two external walls. I suspect also because there's a steady draught through the floorboards and from under the skirting. It was always dreadful in winter for the window panes running with water each morn, but then he'd shut them tight. I insisted he crack them to vent - against giant moans - but that transformed the amount of cond.
All I can suggest for you is to try stuff! Def the radiator foil mentioned by others. Compare bedroom door open with window closed, to ditto with window on vent - what difference does it make. See how much an oil-filled for that magic half-hour before waking time helps the feeling of comfort - I'm sure it will. And ditto before bed if you wish. These short bursts won't cost much to run. (I bought the cheapest o-f at Screwfix - £50 - for when bro was convalescing with us last winter. Worked a treat. A plug-in timer will keep use manageable.) 
And make sure she has a magic duvet