You guys needs to get together and write to Derby Cycles about this and seek some assurances that the ongoing motor issue will be covered after the warranty expires.
From what I read on this thread and hear from a friend, this fault has existed from day one of purchase, laying dormant at first, but then materialising after a relatively short period of time. The motor is then replaced and so the cycle (excuse the pun) begins again, eventually leading to a second or third motor replacement. Not only is this inconvenient, but you are also ticking down to the warranty expiry and a £600+ repair bill for the next motor.
It appears that the frequency of motor failures is directly related to the amount of use. The more you use the bike, the sooner the motor will fail.
On a bike costing £2000 to £3500+ this situation is not acceptable. In the first instance, those who have had 1 or more motor failures under warranty, should seek, in writing, exactly what 50 Cycles intentions are if it fails again within 4 years of the warranty expiry. Regardless of the manufacturers warranty, the SOG act gives you up to 6 years if the product purports to be of superior quality and has a price tag to match.
If 50 Cycles start acting like clowns, which they are very likely to do under these circumstances, you need to approach Derby Cycles directly. Ask for the same warranty assurances (6 years). Given that this problem usually appears early on in the bike's life and continues through the first two years, you should succeed with this. Write everything down dates, time, who you spoke to, what was said & what happened. Also demand a £0 invoice for warranty repairs to the motor.
Citizens Advice will give you information regarding likely cost of SOG action if required. I think it's relatively easy and low cost. Probably a County Court job. Personally I think Derby Cycles is a good bet, I had a positive response from them over another issue.
The main outcome you need is that assurance that this ongoing motor problem will be addressed beyond a two year period.