central locking trunk/boot problem

Hi everyone, Im new here, looking for some advice.
I bought a C70 last year. I like it but it doesn't like me.
Its now started this thing with the boot. It wont let me in. All other doors unlock as usual when I use the button on the fob. But the boot stays locked.
It stays locked untill I give up and get in the car. As soon as I turn on the engine. The boot not only unlocks but also pops open, so that I have to get back out to shut it.
When I get where im going and turn off the engine. The boot pops open again as it turn the key to off.
Im presuming its electrical problem of some kind otherwise it wouldn't open at all would. However I have no idea abt electrics.
Ive already replace battery in the fob. and Ive already disconnected and reconnected the battery, both to no avail.
Anyone had similar problem ????
Nige