Keyfob not working...changed battery

[willd]

Hi, I don't know if anyone can help, but....

One of my keyfobs was seemingly getting low on battery power. So I've swapped out the battery. It was only out for about five seconds...I know from previous cars that you can't keep some out for too long before they 'lose' the code.

Anyhow, the keyfob now doesn't unlock the car. The red LED flashes once, as opposed to twice on my other fob.

I will be honest and say I will be gobsmacked that when you come to change the battery, you have to take the fob into the dealer to get reprogrammed.....surely this isn't the case? On a few forums I've read postings from taking it to the dealer, to turning the battery upside down and pressing some buttons (?), to unlocking the car manually, putting the key into the ignition, and pressing the unlock button (tried this it didn't seem to work, unless I didn't do it properly).

Can anybody help? Seems absolutely ridiculous that only the dealer can do this. It's only just come from the dealers (yesterday) after having £3750 (circa $5500) worth of work doing to it (replacement cats, roof gearboxes replaced, brakes lines replaced, brake servo replaced, new O2 sensor, two new rear tyres, MOT (yearly department of transport vehicle check), new micro-switch assembly for the roof catch, all the exhaust clamps and heatshields replaced, and separate from the dealer I had the exhaust modification done - the bypass pipes*. So I'm a little peeved I have to go back to the dealer (25 miles away in central London) to get the sodding keyfob done now.

Any help appreciated, thanks!

*Exhaust modification - absolutely worth every penny. Really improves the sound of the exhaust, it's sweetest around 2.5k to 3.5k revs, no resonation in the cabin, sounds much more like the more recent 911s.