If it's a dichroic lamp - it'll probably need 12v to run - and probably more like 11.5v for the lamp to last a while, and remain colour stable (although there ARE 240V dichroics, so do check). So a 240VAC to 11.5VAC transformer may be the go. There are electronic switching versions of these, commonly used for domestic lighting dichroics, that might be too "flickery" for an enlarger - I suspect a transformer based version might be better for this application.