First Gear will not engage properly when coming to a stop

Have a 2007 C3 1.6 Auto PRND Gearbox. I bought this car 2 years ago, when I first got it the car had this problem. Once it had heated up when I pulled to a stop it will not engage 1st gear (its kind of half engaged like the clutch hasn't released correctly). The only way to move is to flick the lever to manual select and select 2nd gear. It will then engage this gear (sometimes with a bit of a clunk) and you can take off. Once travelling it happily changes back and forth between 2nd,3rd & 4th gears. This didn't happen every time and after a few months it ceased completly. Over the last month it has come back with a vengance. It doesn't seem to happen when you start out , only when you come to a stop. Reading a few forums there is a suggestion that this has been a problem and that rebuilding the Gearbox ECU may fix it. Anyone have any suggestions...

There are a lot of things that can go wrong with the AL4 gearbox and I may not cover all of them. Some can be tackled by the DIYer some I would leave to the professionals.

A DIYer can try changing the gearbox oil. At some point it's possible it has been changed for the incorrect fluid. The correct fluid is Mobil ATF LT71141 or the equivalent from Citroen.
The next is possibly adjusting the manual valve. I have a blurb HERE At the end of the post.
Speaking of that link, The next thing is the electro valves. The 2 modulator valves mentioned in the linked post are another cause of problems in the gearbox. This should only be attempted by a serious DIYer or a professional.
The next are the "other" electro valves on the valve body. These valves control the changing of gears and the valves are either ON or OFF. The modulator valves mentioned above, pulsate at a high frequency and tend to wear out more than the "other" valves.
The brake bands. Old age and old oil, can damage the brake bands. The brake bands lock parts of the gearbox in place. Damage causes them to slip. They are buried deep in the gearbox and require the gearbox to be stripped down.
Wear of ANY of the components of the gearbox may cause the gearbox to function incorrectly. Diagnosing exactly what is causing your problems without pulling the gearbox apart is something for an expert.
I have outlined some things that you might try but without professional help it's just hit and miss.