We run 2 excel machines all the time, never had any problems using extension cables, remember this machine is pulling 6kw, it needs a good quality 13A extension lead, not 10A or 5A bought from B & Q. As stated the lead needs to be fully unwound to prevent overheating, but this is standard practice for any extension.
The advice of getting a TM for the sake of a good quality extension lead is slightly OTT.! Although I am not against TM's (I will probably get a prowler in the near future) our machines run day in day out with excellent results and numerous repeat bookings. I personally think the Excel is one of the most reliable and robust portables on the market.