Thanks for the replies guys, I have now bought the car and im chuffed to bits with it
I got it for a really good price and so i have money left in my budget to get replacement fobs and sort out some other minor issues...(currently minor but who knows!)
Stop - before you buy a replacement...
Within half an hour of getting my 156 home after purchase, I took apart the key and repaired it. This is the strange curved-shaped key with the flick-out blade, door lock and boot release buttons - is yours the same? Also, do some searches on this forum - someone documented the process well.
The Alfa Romeo badge prises out to reveal a small Philips screw. Then the casing pries apart. It is made of a soft, flexible plastic. My door lock/unlock button was 'stuck down' and the tact switch (a surface-mount electronic switch) had fallen off the circuit board - was loose inside the key. A couple of moments work with a soldering iron reattached the switch (it's plain to see where it goes).
As for the button, which is moulded into the casing, I used a heat gun to heat the plastic VERY carefully - when it became almost too hot to touch, I pushed the button 'up' and let it cool. That restored the original elasticity of the button (it needs to pop back up after it is pressed).
The battery is a CR2032 - very common 3V lithium coin cell - CR2025 fits as well if you don't happen to have a CR2032 on hand.
Sorry, no pictures as this was all over from start to finish in less than half an hour. It is such a thrill to never have to stick a key in the door again, and at no cost for the repair