Alltec use blue 16a connectors on their machines with a 2.5mm connection cable that will take 13a comfortably. If I need to extend the cable I use a caravan cable that is rated at 16a and is orange. A 50 mt cable is about £20 with a male & female coupling. An extra female coupling is about £3 so cutting the plug off, cutting the cable in two and fitting a 13a plug on each and a 16a socket on the other gives two 25m extentions rated at 16a which does not give too much of a voltage drop over the distance. I can then run heater and vacs at same time.
Nearly all decent twin vacs with 135 or above pumps use at least 13a and usually 14-14.5. This is why plug fuses last 9-12 months-- they slowly cook.
It is imperative to fully unwind all extensions and not leave them even in a loose coil as the magnetic forces will make them overheat. I have seen a fullup 13a extensoin melted into one lump with the middle smouldering in less than 10 mins.
If you are using a booster box and heaters try to find a cooker point to plug the main mc in. This will take some load off the normal ring main. These are either fused at 35 or 45 amps and a mc at 13 + heater at 13 + booster at 8-9 and your vacs, agimat, and the lady's kettle for the tea can soon overload the circuit. The trip is bound to be under the stairs behind all the usual junk that breeds in dark places!
Happy days
Trevor