Lee Andrew,
just put it down to gaining a bit of experience and take your time quoting next time, dont be rushed by anyone.
I would clean it regularly because you may get other work from it, and look for other things to increase your price like signs etc.
After about 6 months increase the price, once you have done a good job and arrive on time, you should have no problems at all.
I have miss quoted in the past and have cleaned the work, but just lastweek i turned a job down immediatly as they wanted a price straight away and i wont be rushed. The job itself would have been about £1,000 per clean but because i wouldnt be rushed i told them i needed to look properly at the job rather than pictures.
By keeping to your values and wanting that time to quote correctly, you will not go far wrong in the future.