http://www.holden.co.uk/hvc-home.htm
is the website where I got my headlamp. The Lotus original headlamp glass protrudes further than the replacement, so I had to pack the gap with string or rubber strip. Also had to grind off one locating lug on the headlamp(expected this anyway- it happens on a lot of multi-fit replacment lenses). Unfortunately the headlamp I got (think it was the Saturnus ones) have slightly convex glass, regular Elan lamps are flat. However they did cost me only £21 each so a bit of a saving. They also passed the MOT without an issue and I replaced both even though one was ok.
It might be worth phoning up (they didn't respond well to emails) and asking if any of their budget 5&3/4" headlamps have flat lenses.
When you have a look at their list of headlamps choose the category Light Units 5 3/4" as they have other more expensive ones.
However for £45 it might be better just to choose the Lotus ones for peace of mind, although my replacement ones are Halogen as well.
Rusty screws, mine came out of the headlamps ok, use a good screwdriver and some penetrating liquid on the screw threads if you can. I had to replace a couple of screws on the retaining ring as these were rusted and damaged on removal.
'Who is to know what is possible until one has attempted the impossible and suceeded'