Question: Morco water heater that wont heat!

The limescale is a possible answer. If the boiler is lighting correctly as you say it is usually low water pressure or lack of gas that causes it not to function properly. I think it's unlikey to be the gas as it is lighting in the first place but it would be worth checking that you have plenty of gas in the bottle first.
Failing that I suspect that you have probably got a restricted water supply problem. Assuming you water pressure is fine then this could be due to limescale. One cure for this could be to remove the water jacket - fiddly but not too difficult and immerse the whole thing in a descaling product. If that doesn't work then you are probably looking at a new water circulating jacket. Much cheaper than a full heater and not that difficult a job (escpecially if you have previously removed the old jacket for descaling as you'll know exactly what's involved).
I've replaced/repaired water jackets quite successfully on this type of heater with little more than a decent spanner, screwdriver (usually a pozidrive) and PTFE tape to ensure water tight joints when you re-assemble everything.
It's obvious I know but don't forget to turn the main water supply off before you start - you could end up getting an unexpected shower otherwise.

Yes, I agree with Mike that it could be because of two problems. One could be gas supply and second, could be an issue with your water pressure. In this regard, you can conduct thermocouple testing and yes, it is not much expensive. I also found this guide https://highperformancehvac.com/how-troubleshoot-standing-pilot-gas-water-heater/ as a better solution to know where is the problem. However, it is better to contact an HVAC contractor to fix further issues.

Bad gas valve could also be a reason of this problem but you should avoid proceeding with any danger and better to go with expert services.