It would be worth checking that the boot switch is not at fault as this seems to be a common fault (the internals get water inside and corrosion sets in)
To check this, remove the 4 torx largest screws which hold the shroud (which houses the bootswitch and number plate lights) and carefully remove...be careful not to damage the seal as this is supposed to keep water out! It might be stuck well on but it should come away in one piece with a bit of effort and gentle persuasion.
Once removed, (make sure the car is unlocked otherwise the boot wont open!) disconnect the plug that you are faced with. Get a piece of wire and short out two of the pins on the plug (I cant remember which 2 but there are only three pins you can short so you will soon get it!). The boot should release, assuming the car is unlocked. If that happens then it is the boot switch that is at fault.
You won't be able to get to the switch because everything is closed in as you will see. I found that one of the bulb holders on mine was suffering with corrosion - I basically sprayed WD40 all over the unit and cleaned up the bulb holder. Also worth checking the pins in the plug and giving these a squirt of oil too. Everything working fine for me again.
A new part from volvo in aberdeen (just replacement internals) will set you back around 110inc VAT. You might be able to get a replacement part from a scrappy but apparently this is a common fault with Volvo's so prob not worth it because you can't see the condition of the internal electrics which is probbably where the problem lies.