OK. Firstly, I am not a lawyer.
In regards to speed guns used by UK Police, there are certain strict regulations that they must follow. AFAIK, the gun has to be fixed (via tripod or other fixed means - as handheld guns can produce inaccurate readings as the operators hand slightly moves when taking the reading, thus invalidating it). Also, the speed gun used must have been tested that day, and if you ask they should provide you with a certificate of approval. The PC operating the gun must be wearing high vis clothing - he must be clearly visable - not hiding in the back of a van in darkness. There must be signs (showing that "safety" cameras are in use - the little black speed camera sign).
Also you may want to check to see if the signs along that road are as "prescribed" by regulation. For example, the first change in speed - a new sign, eg. from 40 to a 30 (the first 30 sign), if placed on a primarly or trunk road, where the sign shown in diagram 670 (statutory speed limit) is placed within 50 metres of a lamp powered by electricity which forms a system of lighting where 3 or more lamps are placed no more than 183 metres apart (differs in Scotland), then the sign shall be illuminated by means of internal or external lighting, blah blah blah.
All this traffic sign business can be found in the Traffic Signs Regulations and General Directions 2002 (Statutory Instrument 2002/3113) -
http://www.opsi.gov.uk/si/si2002/20023113.htm
If you find the signs are "non-prescribed" then they are void. Infact they are unlawful and highway authorities that errect signs not prescribing to regulations are "acting beyond their powers". Non-prescribed signs can be classed as "obstructions" of the highway.
There's a very nice piece of case law, the case of "Davies -v- Heatley [1971] R.T.R 145" which basically states that if a sign is non-prescribed then contravention of the "sign" cannot constitute an offence even if the "sign" is clearly reconisable by a reasonable man as a sign of that kind.
The case of "MacLeod -v- Hamilton [1965] S.L.T 305" also deals with the legality of "non-prescribed" lines.
As for the factual ins and outs of speed guns, i'm not 100%, but i'll have a look through my archives to see if I can fish out any case law which may help you.
As you said though, no camara, no evidence, and that's your best argument - "I was doing the speed limit officer unless you have evidence to the contrary".
BTW, I do not condone illegal speeding, only safe speed.