Sam,
Do those only come in 75 watts? If so I just can't see them heating a 6' cage. They are the same principal as the Radiant Heat Panels we use in the states and I don't see a 75 watt RHP heating a 6' cage. I'd guess you'd have to use two of them, I don't know. If the room where your boa cages are is heated one might be fine.
You should ask around in the UK as I'm sure they will run on 220V so maybe that makes a difference. I don't think so but you should check.
Also where is the best place to put the thermostat - the heater is on the ceiling and I always keep the stat directly under neath at the bottom of the viv. I have heard it is possible to put it a bit further down the viv to ensure the gradient is maintained and set the stat a little lower. Im worried this will make the warm end too hot.
This is a tricky question. It really depends on what the cage is constructed of, the amount of placement of the ventilation, and the temperature of the room where the cage(s) sit. Even with that information it's a bit of a guess.
Really it just takes a bit of experimentation on your part. And understand that what works in the summer might not work in the winter.
In general I would say use the Tstat to creat a usable basking area and then increase room temps and/or insulate the cage to create a suitable temperature gradiant. You can also partially cover the cage ventilation if you have too much.
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Current snakes:
0.1 Gonyosoma oxycephala - (Silver/Yellow)
1.2 Gonyosoma oxycephala - (Green)
7.6 Gonyosoma janseni - (Black)
0.1 Gonyosoma janseni - (Black & Tan)