I had previously made the cylinders and motion but not actually mounted them to the chassis. After pondering how to do this for some time, I settled on the idea of a removable unit to mount both cylinders. As you see in the first picture, this is made from a piece of thin PCB, which will be horizontal on top of the chassis block, held in  place by the body fixing screw. At each end I soldered pieces of brass tube that will hold the previously-made cylinders, increasing them to something like the correct diameter at the same time. Usually the cylinders would be mounted in a vertical plate going across the chassis, but this wasn't possible with my solid chassis block design. This photo shows the unit before drilling the fixing hole:

As you see in the next photo, I filed a recess in the top of the chassis block to locate the PCB:

And here is a view of the underside. Note the gaps to maintain electrical isolation. The cylinders are just held in here by friction, but I subsequently glued them in with Araldite.

The cylinders in place with the motion attached:

They're not actually fixed in place until you screw on the body:

Now it's beginning to look like an engine! The pistons still need trimming to length at the front, and I need to tidy up the fronts of the cylinders somehow - perhaps with some thin discs stuck on.

It was very satisfying to find that (with the motor detached) the loco rolls freely up and down a tilted length of track. But I have to admit that this wasn't achieved without a bit of effort. Firstly, the coupling rods were too tight, and I had to gradually open out the holes until the chassis would run freely. Then I found that there wasn't quite enough clearance between the slide bars and the rods, so I had to unsolder the brass tubes from the ends of the PCB and shift them outwards a bit further. Luckily there's no scale drawing looking at the loco front-on, so I had some freedom in the side-to-side positioning of the cylinders. Even so, I won't be able to fit crankpin washers on the front wheels.  In fact, it still won't run up and down with the body properly in place, because the other crankpins haven't yet been trimmed and just foul the bottom of the valance. But it's getting there! The next job is to sort out the motor.