Thanks to everyone for replies, first of all I have to agree with wirepuller on his quote that when running a business it is simply unrealistic to carry out testing before quoting a customer for a board change. So well done to you for your honesty. I do, however always carry out a visual check to see that the condition of the wiring is in suitable condition to re-terminate into a replacement consumer unit. There have been many occasions where I have politely declined to replace the consumer unit stating that an upgrade of the wiring would be required
Secondly I always provide certification post board change. On this occasion I had started the board change late on in the day and encountered this problem late on. I, of course, restored power to the property temporarily and then first thing in the morning revisited property where, thanks to some of the more helpful comments, I managed to quickly trace the fault to a junction box below the floor which had a damaged cable resulting in a N - E fault. This was quickly rectified and the RCD reinstated, installation tested with satisfactory results.
By the way I would like to state that I am in no way a Electrical Trainee! I have been in the trade for a long time and have completed the 2391 and have been very involved with electrical inspection, testing and certification since. I merely posted a problem I was having with a view to receiving some feedback with some ideas of where the fault most likely would be. I must be under the illusion that this was the purpose of the forum. It did seen though that as soon as some criticism was raised a lot of other forum members jumped on the bandwagon, slating me, presuming I was a Electrical Trainee/cowboy that had to clue or inclination on testing. It does seem that some of the members revel in jumping on these situations as soon as the opportunity arises. To those members I do apologise that I am less gifted than yourselves!!
I wish I did have the time to carry out a full test before quoting for consumer unit replacement but the trade is on its knees as it is and to be asked to carry out a board change these days instead of customers getting the work done as a homer is a rare occurrence in these parts