Fair play Mo, we have on here
Climate experts
Plane crash experts
Fiscal experts
Covid experts
Virology experts
experts for everything depending on what is topical, unfortunately
no plumbing experts!!!.
Other than gas i do my own plumbing
Hot water tank ?, it's all condensation boilers nowadays in blighty except for the climate nutters.
Is that valve a Caleffi 526 model ?
https://www.waterware.co.nz/products...20mm-x-700kpa/
If it is, I'd have a scan on line for the Operation and instruction manual, the manual on the site linked only has the installation instructions, the OIM should give a parts breakdown & assembly instructions from your picture I'm presuming that it's leaking through the valve and not where the grey elbow is screwed on ?