I'd suggest a reputable company to come and programme it for you, they will need the original installer code or they can crash and reprogram it. Get it serviced at the same time if it's not been done for a while (12m is recommended) and this will ease the pain of the call out to a non maintained system.
May also require a new back up battery if its older than 4 years.
Hi Pete, we don't give out defaulting instructions or engineering codes for obvious reasons sorry.
Any decent company should be able to carry out the work for you on an ad hoc basis, have a look for a local reputable firm.