what csa is the flex cable to an immersion heater, in the UK?

[batty]

Indeed....Also ...

Am I right in thinking some heat res imm heater flexes don't have copper strands?

Or are they just tinned copper down their length?

:|

Not sure what its made off but my butyl is not copper coloured.

Quick look its tinned copper

http://www.eland.co.uk/electricalcable/rubberflexiblecable/cable73/318-tq-cable-h05bn4-f.html

 

[SLIPSHOD & SLAPDASH]

Re ferrules. I have never done or seen that. My view is they are another unnecessary joint. The manufacturers instructions even show you how much insulation to strip off the flex before inserting under the screws, the screws that have been designed to clamp a flex.