Firstly may I apologise if I'm preaching to the converted...
I was alerted by an irate bus driver that my right headlamp was out - so dutifully I called MX5parts for advice before purchasing and having it replaced...was advised I needed to replace both headlamps, at a cost of £24.95 each, plus the postage and a day's wait at least. So rang my mechanic [who conveniently was leaving the country for week], who gave me far better advice which I now feel I must pass on to all Mk 1 MX5 owners - don't waste your cash guys!
Simply pootle on down to your nearest Halfords, and ask for the "Sealed Beam Unit with Pilot Aperture R7014", for which they charge the princely sum of £6.99, and with a 10 minute wait while they fitted it at a measly £4.99 - bargain! No time wasted, and 40 smackers richer...
And they appear to have loads in stock too, as particular minis take the same headlights.