If you are not satisfied, you will need to take them to court.
Most technical delays are subject to 'extrodinary circumstances' when it comes to this European regulation. The airline will provide specifics for the flight in great detail, and in general even though the regulation favours the customer, if it reaches court level, the airline will win.
Jet2 will have to provide information regarding the component that failed on that particular flight. They will then have to prove the shelf life of that component, i.e. 10,000 cycles. If the part failed at say 5,000 cycles then the Airline is not held responsible. However if it failed at say 8,000 cycles then there is a possible case that the airline were expecting this to happen. Most of the maintenance on the fleet over the last 12/24 months has been to replace parts that are approaching their shelf life. If you are talking about the delay you previously mentioned to me, this particular flight delay was caused by a new component from Boeing failing.
The court will then look at the welfare issued by Jet2, no doubt approx £5,000 mark for that return flight. They will also look at what other measures Jet2 took to minimise the delay, i.e. the use of a sub-charter or positioning in of a standby aircraft, re-shuffling of the fleet to say delay 2 flights rather than one big delay etc.
When investigating each flight, Jet2 make a decision based on the above, i.e. if they would loose in court, they pay the compensation as it is cheaper than the legal fees, however if they have a strong case they will not pay and will be more than happy for you to take them to court - however, should they win, they are not liable for any fees.
You really should believe what they are saying, they are legally bound to tell the truth as it would not stand up in court and would cost them hugely if they lied. Whilst this does not give you the answer you want, it might give clarity on what you need to do.
With regard to this EC/261, much like the PPI claims, legal firms have seen huge £ in helping customers claim for delayed flights. By all means try claim, but the airlines will provide explicit details to stop them paying out millions in compensation as it simply would bankrupt most carriers. Personally i'd weigh up is it worth the legal battle and most likely £££££ in legal fees, for very slight chance of getting £300 compo? No.