Have you had a look at the (workshop) manual?

What happens if you manually select 1st gear, or if the car is
stationary and selects 1st by itself? From your description, I get the
impression it goes nowhere, and that manually selecting 2nd forces it to
2nd and allows you to get moving?

If so, we need to find the part of the gearbox which is unique to first
gear, the thing which can fail without affecting any other gear.

There are essentially 6 "bits" in the gearbox which hold or let go at
the right times to cause the right gears:

Two clutches:
1) Forward clutch.
No forward clutch, no forward movement. Very common failure, relatively
simple to fix.

2) Top and reverse clutch.
No T&R, no 4th, no reverse. Simple enough. Rarely fails and is a bastard
to get at.

Three bands:
3) Reverse band.

4) Third gear band.

5) Second gear band.

No prizes for guessing, if a band fails, the corresponding gear doesn't
work.

6) First gear one way clutch
What do you reckon?

If you actually consult the diagnostic procedure for the auto, basically
you'll get:

No drive in first gear, automatic or manual: 17, 18.
...
17) Check freewheel support dowel bolt sheared.
18) Check the one-way clutch unit.

Whilst it's not a job for the faint hearted, neither is it absolutely
beyond the competent home mechanic, but you WILL need the manual and
somewhere clean (no draughts and dust) to do it.

There is no "selector" as such. It's an epicyclic gearbox, completely
different to a manual, and it engaged gears by stopping various parts of
the gearbox.

When the forward clutch is engaged, you get first gear. By engaging the
2nd or 3rd bands, the freewheel clutch freewheels and instead you get
2nd or 3rd. The top and reverse clutch causes 4th. The ONLY component
which is unique to first is the "freewheel" or "one way clutch" (same
part, two names).

If it's the dowel bolt then you won't have to pull the gear train out of
the box and it is fairly straightforward (once you've got the converter
and it's housing off the motor and box!). It's probably no worse than
removing the flywheel and housing from a manual Mini.

I would think second hand MiniMatics should be cheap enough in the UK to
get a complete motor and box from somewhere else for less pain and expense?

Graham

Hello everybody from Italy!
I still have a little problem with my autobox (1992 rover 998cc):
slipping in upshift from 3rd to 4th, aleatory behavior of second gear.
All these problems (as Graham kindly said replying a my old post) are
caused by low oil pressure and/or defective forward clutch, i'm still
not completely sure about the cause, just because i couldn't execute
the oil pressure check: all the gauges i found, don't fit to the
threaded pressure port, and i did not found any suitable connector.
few days ago, after a carb cleaning, my mini started to make a new
horrible noise: "grrrr-skreek" during or after a quick downshift on
stopping after hard braking. (hmmm... I've set the idle speed very
very low because i don't like the car "crawling" at idle..) Anyway,
assuming the worse scenario, i would have to replace oil pump and
forward clutch, i have factory workshop manual. Where can i get
rebuilding kit for AP transmission and OE or aftermarket oil pump? At
rover prices are very high! Thanks in advance for replies and best
regards!
Ciao
emilio