Lockshield valve leaking - what to replace?

[keyzjh]

Hello, to do some plastering work, I've isolated and drained one radiator from my pressurised central heating system with combi boiler.

After reconnecting it and re-pressurising however, the lockshield valve is leaking at the connection to the radiator.

I removed it again and noticed this old O ring from within the valve.

Questions:
1. Is the O ring the reason for the leak?
2. If so, do I replace the valve or O ring? I can't seem to find like-for-like replacements for either, as the diameter for the thread is approx. 1 inch
3. Is the procedure for swapping out lockshield valves similar for TRVs?

I assume this new valve doesn't work as the nut is on the existing radiator.
Example: https://www.screwfix.com/p/white-angled-manual-radiator-valve-15mm-x-1-2-/33920?ref=SFAppShare

Any suggestions and keywords for the product I should be looking for?

EDIT:
On a product page, is the connection to the radiator the 'inlet' or the 'outlet'? I may have misunderstood all the spec sheets!

 

[CBW]

If it’s an o-ring rather than an olive to seal, then yes, you’ll need a replacement O-ring.

 

[keyzjh]

If it’s an o-ring rather than an olive to seal, then yes, you’ll need a replacement O-ring.

Do you mean copper compression olives? I've run a finger into the valve and there isn't anything else in there other than this deformed rubber ring

 

[Bod ...]

It's getting the right Oring that's now the issue

 

[CBW]

Do you mean copper compression olives? I've run a finger into the valve and there isn't anything else in there other than this deformed rubber ring

Doesn’t matter the material, as you can get brass olives. As per @Bod ... obtaining a replacement could prove tricky, probably best replacing the valve/tail set.

 

[keyzjh]

I'll try out these assorted sizes rings. I'm not quite sure if it's 18mm or 20mm diameter as it's so deformed.

On a side note, every time I drain this single radiator I will inevitably lose 2-3L of water with inhibitor mixed in. Do I need to add more inhibitor when I'm done?
For context I only have 7 radiators total and a whole bottle of MC1 had been added about 6 months ago.

 

[keyzjh]

Doesn’t matter the material, as you can get brass olives. As per @Bod ... obtaining a replacement could prove tricky, probably best replacing the valve/tail set.

Do I need to replace the radiator as well? I can't seem to find a similar sized valve to this seemingly very old one.

 

[CBW]

Do I need to replace the radiator as well?

Not at all.

I can't seem to find a similar sized valve to this seemingly very old one.

The tail should be 1/2” so it doesn’t matter if you replace it with a 3/4” x 1/2” or 1/2” alone, as long as the dimensions are similar. You may need a radiator Allen key to remove though, not expensive either.
Screwfix

 

[mcprinter]

Toolstation do a twin set of radiator spanners/allen keys

Monument Twin Pack Radiator Spanners | Toolstation

Zinc plated offset handles for radiator jobs. Monument 3in1 and 4in1 Radiator Spanners Made in the UK

www.toolstation.com

The Screwfix one didn't fit one of my older rads

 

[keyzjh]

Not at all.

The tail should be 1/2” so it doesn’t matter if you replace it with a 3/4” x 1/2” or 1/2” alone, as long as the dimensions are similar. You may need a radiator Allen key to remove though, not expensive either.
Screwfix

Am I right in understanding that buying a new valve also includes the tail (as annotated in picture)?
So to get this working, I need to remove the existing valve from the pipe, and existing tail from the radiator?

The pressure in the loop is about 0.5 bars now, would I need to drain down the whole loop to replace the valve/tail?
I've seen a few videos where draining down isn't required to replace TRVs, I would think it's the same for replacing a lockshield valve.