MARK HARDACRE
Anything is possible in theory - Just need to work out max demand, and pick the correct size cable. SWA (Steel Wired Armour) ain't cheap. However it is necessary in this situation. 8.5kW / 230V = 36.9A (Amps) so your cable needs to be rated for this as a minimum. Generally Voltage drop is only a factor for runs over 100M Including the final part of the circuit. Generally (There is too much detail to go into). 6mm 3 core SWA will give you 46A 'to play with' you could cheap out on 2 core and use the sheath as earth. I'd rather not. Currently 100M reel of SWA 6mm is £253.00. 4mm is rated at 36A however there are de-rating factors to account for. 6mm is a safe bet.