My wrenches were both around £35-40 so I'm wondering if it's actually cost-effective as the only price I've found online (Sealey) was about that much, so I might be as well to just buy new ones! Any thoughts?

And are the deflecting-beam types better?

I prefer the click type. Although I can't actually remember the last time I actually used my torque wrench. Not sure if I even know where it is

Re-calibration should only cost about £10 tops, it's only ten minutes work - including the certificate!

If you have more than 1 you can check them against each other.
If they read the same or close, a good bet they are OK.
If not you need a third one to prove which is correct

Cut a short length of hexagon bar[an old allen key] and fit it into 2 sockets the same size on each torque wrench.
Set them to the same readings say 5, then 10, 15 etc.
Pull them against each other to see if they both click off together.

You can adjust the setting until they do, that will tell you how far out one if them is, hence the need for a third one.

That's a clever technique Dave,

I've seen a few online that involve suspending weights off the end of the wrench.

However I have 2 different types. What I do know for certain is that it'll be out of sync now as I couldn't help taking a peek inside and it's all come apart! There's a tiny pin that I'm not sure where it lives, and the spring needs compressing (I think) as I can't push it in by hand. Never mind.

Buy a new one and then check your working one against it.
Its useful to have different ranges.
I have 3 to cover different ranges, up to 10, 50 and 150ft/lbs

You can check with weights if you happen to have some, as torque is distance x weight, but easier with another wrench.

If you have an airfield nearby, you could enquire with the technical staff there. Most airfields will have a calibration rig on the wall somewhere.