Don't think any other atco's have chipped in on this yet so I'll chuck in my two penneth.
topcat450- Coventry don't have secondary radar so you could tell em you were squawking anything you want they wouldn't know any better!! They issue a conspicuity squawk (at the minute its 0250 for FIS traffic) which just shows us people who do have seconday that that aircraft is talking to Cov should we need to effect co-ordination. they cannot identiry on the basis of this squawk nor can they verify mode C readouts.
QNH1013-very nice of you to say such nice things about us. We do try our best to accomodate non transponder aircraft and given the time we can identify an aircraft by several different means not using SSR.
For information (and without checking the books so any other ATCO's please don't pick holes in minor mistales!!)
-over a nominated VRP provided you are flying below 3000ft and with visual reference to the surface providing the VRP is marked on the radar map (all the East Mids one are) so if you are at 2000 ft over head Trowell, and we see a blip there, bosh identified.
The other commonly used one is, as QNH1013 says the turn method. We will have a fair idea of of which blip you are from you position report and the DF, we can check this by giving you a turn of at least 30 degrees to check the blip we think you are complies.
The other thing we can do is not identify you at all and just give you what we call a procedural clearance-that is one where we are not really using the radar. An example of this might be wanting to transit the East Mids eastern stub (bast 1500ft) at 2000ft-no inbounds for 27, no problem clear you stright through and maybe get you to report eat abeam so we get an idea of how your progressing.
I've gone on abit more than I intended, and there are some other means of identifiying without transponders (but they generally involve VOR/DME etc which I think you would all agree would probably be fitted after a transponder!!- although my hand held Comm does have a VOR function......)
Upshot is, certainly at East Mids, don't let not having a transponder put you off calling us, especially if your going underneath our airspace. And if we can we will give you transit. I don't think I have ever refused a transit because an ac has no transponder.
And finally, the sircraft I fly regularly doesn't have one!!