To take advantage of the Octopus Energy feed in tariff I need my MCS Certificate number.
Where can I find this?
MCS Certificate

From the certificate the installer should have given you. We got ours two weeks after installation.
We got a handover pack from our installer pretty much after I’d paid and it was in there.
In short, from your installer
If, like me you inherited a certified system and don't have the certificate, [https://mcscertified.com/]
will provide you with a replacement for about £30 iirc
MCS will only issues a certificate on NEW equipment - as I found out, my system had to be removed & re-installed as the original contractor - Koze group / home screwed up.
I got a MCS / NAPIT approved contractor to fix Everything Then MCS said "we can't issue a certificate on 2nd had equipment" !!
They are a joke , but your hands are tied by them.
Also octopus charge £250 to take on non MCS approved installations - for the feed in tariff !.
Octopus do now accept Flexi-Orb certification after previously only taking MCS, no testing or application fee was required.