Adding 50mm of extra insulation to a 550mm diameter cylinder adds 20% more surface area.

Maybe a 20% extra surface area but theoretically 50mm extra insulation may increase by ~ 100% the thermal R value ....or not ?

debo:

 

cadman:

 

Aredwood: Add extra insulation as others have said.

 

That can actually be counter-productive on 'modern' cylinders though. There is an optimal level of insulation due to the increasing surface area with additional insulation and most manufacturers would be working within that range.

 

 

not true.  the effect you are talking about is for small pipes (<1cm) like get used in refrigeration.  Any additional insulation would help.  A good tool is a FLIR camera and then you can see where you need to an the insulation. 

 

During the 2000s sometime, Rheem increased the outside diameter of their range of hot water cylinders. Reason - Make the insulation thicker, approx 60mm larger diameter. Almost certainly this was done to comply with the 2005 standards that I linked to earlier. So it would be a safe bet that more insulation definitely reduces heat loss from a hot water cylinder.

nickb800:

 

Perhaps add a loop to the cold water intake, so that water runs through a coil in the ceiling space before entering the cylinder. So you're pre-heating the water, and let the insulation help with maintaining heat

 

This will only help when you use hot water during the day. And at night time during winter, the roof space could easily be colder than the water delivered from the council main. As the council pipes would be buried deep enough that they will be exposed to more stable ground temps. Plus you would need lots of pipe to get even 1KW of heat transfer at say 10L/min flow rate.

Aredwood:

 

nickb800:

 

Perhaps add a loop to the cold water intake, so that water runs through a coil in the ceiling space before entering the cylinder. So you're pre-heating the water, and let the insulation help with maintaining heat

 

 

This will only help when you use hot water during the day. And at night time during winter, the roof space could easily be colder than the water delivered from the council main. As the council pipes would be buried deep enough that they will be exposed to more stable ground temps. Plus you would need lots of pipe to get even 1KW of heat transfer at say 10L/min flow rate.

 

You would get condensation forming at night when the warmer inlet water hit the roof space.

To solve the issues you mentioned you would need some kind of large heat sink to expose the inlet pipe to as much warm air as possible and some kind of bypass so the water only entered the roof space when conditions were suitable.

If the OP was going to that much trouble he/she would have to consider some kind of externally mounted solar heater which would be far more effective.