Jim GM4DHJ ... formulated the question :
but what is the looks like a slide lock half way up in the tv adverts...

Mine doesn't have one, so I don't know what it is. My door has a sort
of series of nylon chain links, drive the chain over centre and it
forms the locking mechanism - try to lift the door and you would be
pushing against the over centre 'chain'.

The drive motor is a 24v ac tubular motor and gearbox, that includes
meachanical limit switches, for for going up, one for going doing and
they are very precise.

The control system includes a maximum run timer and a maximum current
sensor - this last is what your problem is, it is set too low.