I live nearby and have used their services 3-4 times for Heathrow airport transfer and stopped it because of poor customer service.
- Drivers were on time +/-5 min, however once they went to a wrong address
- Driving to the airport was often rough and complicated (using back road ?!) making some people sick
- When flight are delayed, there is no mercy...GBP 4 in cash for extra 15 min of waiting (and they stress you about it). One time, my flight was delayed by more than 1 hour. I contacted them to let them know before boarding that the driver should come later much later (I always tell them anyway to come for 1 hour after the flight has landed). When finally arrived, the battery of my phone was dead, so they could not reach me. Once out of the long custom queue, the driver was gone, as 1h 30min had gone since we landed. I expected that, even if I was hoping that he stayed a little longer. However, the definite call-off was the answer from the manager when I called to see if I could get either a partial refund/voucher or some kind of commercial gesture. He was basically bullying me and blaming me for letting his driver down. After 10 min of getting yelled at (is it not supposed to be the opposite normally??), I just hung up and told myself not to use this company ever again.
Hope it helps.