I have recently acquired a Pacemaker in what appears good working order and condition, opening and closing OK. However, on opening it today, the lens/bellows assembly had detached itself from the bedrails. As the workings of this camera are new to me, any help to fix the problem would be great.

It's easy to put it back onto the rails. Rack the focus rail all the way back, drop the bed to it's lowest position, make sure the locking lever on the standard is in the center (unlocked) position, slide the standard back onto the rear portion on te rail. That's it.
I'm a bit concerned about how it got off track. It's not something that just happens. Are the rear rail guides broken? That leaves everything inside the body just flopping around and is caused by improperly closing the camera. The rear (short) portion of the focus rail should be held firmly in place by these short guides inside the body. Any broken rail hinges?

Many thanks for your advice Glenn.
I`ll give it a try.