The radio aerial on the back wing is a Blaupunkt (i.e. Bosch) Autoflex helical whip aerial with alternating wiring direction Standard 0°–30° · 12 V- - code number 7 691 270 001
The mast section of this antenna is available separately with the code number 8 698 114 571 (as per the UK 2005 catalogue). Until recently they were available through any Blaupunkt dealer (got mine from Halfords) and they cost a fraction of what Lotus charge. (Lotus Pt No A100M0170S)
N.B. This aerial has a fine thread M4 x 0.5 bolt that screws in to the aerial mounting on the wing. From my experience Lotus has used different threads on the aerials at different times so worth checking your old one first. If it is different, then you will need to buy the complete Autoflex unit including the mounting block (code 7 691 270 001). Should still be a lot cheaper than buying a replacement Lotus aerial which may not fit your existing Lotus mounting block anyway (the last Lotus one I bought didn't).
Be careful, there is a Hirschmann aerial/mast which the larger M5 x 1 thread. It's an identical part except for the screw thread. Your car could be fitted with either after 20 years.
P.S. For the more technically minded the Bosch aerial is an 'active' aerial. There is also a 'passive' version, code 8 698 141 571.
As of the Mar 2010 this aerial is now obsolete from Blaupunkt (Bluespot)www.bluespot.co.uk
Old stock available from:-
http://www.truckstophawkes.co.uk £26.00 ea plus postage plus vat..
http://www.sjsportscars.co.uk/ £36.78 ea plus postage plus vat..
This may be the your last chance to find some thing that looks OEM
HIRSCHMANN AERIAL 921 186-002 .......Warning....It has a M5 x 1 screw thread
The 2007 Blaupunkt catalogue may help.
As of June 2010, 8 698 114 571 appears on Slovakian, Estonian and other far flung web sites.
As of June 2010 a Lotus aerial group buy, http://forum.lotuselancentral.com/viewtopic.php?f=85&t=17382