sounds like either the central locking motor is playing up or the lock is jamming

or possibly a combination of the 2

the lock is easy to get apart and then all you do is clean it properly to get rid of the crap that gets lodged in it
then you can use some grease of wd40 or the like to make sure it's free from bits of grit etc before you put it back again

as for the c/l motor, just leave it disconncted and lock/inlock the doors to see if it moves or not

from memory the boot one is specific for the boot and it's got 3 bullet connectors on it as opposed to a plug, but that's not to say you won't be able to find a normal slave motor from another car and get it to fit