I tend to use Swells
http://www.reptiles.swelluk.com/rept...mp-243337.html
Dizzy if your tortoise is on a table then I found that nothing short of the 160 watt was any use. The lower wattage bulbs seem to be intended for being used inside a viv or similar situation. But every set-up is different. If your table is very small then it's possible the 160 watt Arcadia D3 would be too much.
The holder for a lamp like the arcadia d3 combi is an E27 ceramic job.