What is this part on my heating system?

ThisIsWeird

The wee brass screwdriver slot facing towards the left. As Grumpy days, try ensuring it's completely horizontal.
There is then a hand-screw-on sealing cap at the very bottom. Check that this is nicely snug, but don't overtighten.
There are washer/O-ring kits available to refurbish such things.
When was it last cleaned?

grumpy_codger

Drain valve - see the link.

Britannia12345

Turning the little screw worked! Thankyou 😀😀😀

ThisIsWeird

Ah, but which little screw?
The side-slot one should stop any drips. The screw-on bottom cap (which doubles as a handle for the side screw) is only an extra security, and shouldn't be 'needed' to stop leaks.

Britannia12345

The side slot one. 
I spoke too soon - it's leaking again today 😭

ThisIsWeird

Britannia12345 said:

The side slot one. 
I spoke too soon - it's leaking again today 😭

Lol! That's often the way 😄 
Don't assume it's the actual valve, tho' - Ie the screw part you are turning. I'm assuming that the whole valve assembly screws into the plastic body, and there will almost certainly be a seal of some sort on there. 
Worth having a good read through the servicing instructions for it, and seeing if something like this is the case.
Or, try rotating - tightening - that whole valve body clockwise - as you look up at it from the floor - by a tiny smidgen.  
If that doesn't work, then get a torch and a tissue, and squeeze the latter around it for a few seconds to soak up any seepage, and then get down and close and shine the torch at it - see from where, exactly, the drip appears. In fact, do that first!

grumpy_codger

It looks pretty new. Who installed it at when?

A new one costs about £60 delivered and you can replace only the bottom part that has to be unscrewed regularly for cleaning anyway.

Britannia12345

It was in the house when we moved in recently. The plumber who checked the system said it was all fairly new and clean but I'm not sure when it was installed. 

grumpy_codger

I meant some possible warranty, but surely in this case there isn't any.

Either live with this or buy a replacement. This is an easy DIY job and cheaper than to call a plumber.

https://www.google.com/search?q=biworld+smart+mag+filter   >>   Shopping (buy exactly the same if you really want this to be DIY)

ETA: this is assuming that you are sure that the leak is a the very bottom, i.e. the water doesn't leak somewhere higher and just drips from the bottom.

ThisIsWeird

Step 1 - ID where the seep is actually coming from.
Step 2 - fix that bit.
Almost certainly it'll be sorted by a fraction-turn tightening, or perhaps a new O ring will be required first.
Tissue&torch. Expect the most likely source to be where that bottom brass valve meets the plastic body.