Measuring for a replacement Combi Cylinder

[Redhotferrari]

Hi Guys,

Could do with some advice on this, apart from 'with a tape measure' whats the best way to measure up an existing combi cylinder (indirect) for replacement, and if I can get the same one will it pretty much be a straight swap except for maybe altering a few pipes?

Just worried about doing a rough measurment and ordering an incorrect expensive part!

Also, the existing one does not have a drain cock, what the best way to empty the old one?

Thanks in advance for your advice.

 

[gas_man_gaz]

ive got one in the hutgoin spare