I am no legal expert, but i have a bit of third hand experience of this...
Right, firstly, they presented an 'expert witness', of what department in the police or otherwise did he represent and what qualifications does he have to be able to be an expert witness (this is just a general question - i am not doubting that he qualifies to be called an expert).
Secondly, this may have helped your case if you had a policeman friend, i'll tell you why basically my Dad who i have the third hand experience of this through, received a speeding ticket through the post too, and challenged it on the same basis - the camera recorded his speed wrong. What my dad then did was go to the scene where he was snapped and measure the distance between the lines on the road as they have to be set, i think he also got hold of the calibration records for that speed camera. It all proved to be wrong and my dad challenged it. The irony of it is, he himself is an expert witness for the police (CIU - collision investigation unit - goes to bad car crashes and assesses the scene, who to blame etc. [nothing can be an accident anymore you see]) Maybe try to get a copy of all the phtots taken of the car alledgedly speeding, go to location measure them up and get someone good at maths to work it out speed vs. time. My dad managed to get his fine overturned, they had to go back to recent people caught speeding and re assess and the road lines were re painted accurately.
Thirdly, setting the cruise control speed at 35mph is i suppose a safe thing to do. Cars ALWAYS read the speed at around 3-4mph higher than you are actually doing - if you have a sat nav then you can see this for yourself as a satnav is more accurate. Speed cameras are also set at 10%+2 to allow for error. So a 30mph speed camera is set to 35 mph and you have to be doing 36mph before you get snapped. So realistically (and i am not condoning it) you can do around 39 on your speedo before getting snapped. You say he was doing 35 on the cruise control - this will have being roughly 32mph.
All magistrates take into account earnings - when i was in court for Drink driving, my finances were taken into account and i was fine very little £100 + costs as i earnt that little, same with the reform course i chose to do £140 i paid as that was what my earnings stretched to.
I think you should take your case to an independent complaints commision, they will be able to analyse everything professionally. You should be able to ask for extra time due to financial hardship - however this has to be proven - you know saying you can't afford a meal out one saturday is totally different to not being able to afford to pay for the Gas bill or food for the table. Asking for extra time cannot make you liable for an extra fine, yet just avoiding paying can, you need to make someone aware of your situation. Bear in mind court fines are classed as PRIORITY debt and should come before such bills such as NON PRIORITY debt which includes things like credits cards, unsecured loans and overdrafts.
I do find the £2000 court costs a little extreme... He should have had it done by a magistrate, not a district judge and court costs shouldn't have really exceeded £100. This is not a professional opinion, just experience, fines are means tested, court costs are not i don't think. I hope this helps and isn't too long