If the '20' is in red, it is not in compliance with the Traffic Signs manual Chapter 3, which states numbers should be in black. It is therefore not a 'real' sign and you can ignore it. Having said that, sign manufacturers do make mistakes and it could be just that (eg I was on a job once and I knew something was wrong with all the traffic light signs round the roundabout. About an hour later the penny dropped...the traffic lights were upside down! It happens!). If there are no 'time plates', the 20mph zone is in force all the time. Might be worth ringing the council to check it is an official speed limit and not one some do gooder has put up. If it is 'real', tell them the signs are wrong!