Thanks again guys - you are very kind.
Nick, thanks. Given what you have said about these castings' "quality" and wih great respect and deference to your experience, I feel must have come across some truly shocking castings in the past and treated them as the "norm"!
O.k. my experience is quite limited - I have really only built two and a half engines from castings up to now and compared to what I have been presented with in those, I really thought these weren't that bad - being reasonably "true (ish)" and with no knobbly accretions.
Maybe I have made a mistake in machining all over, but once you start doing shoulders and such, bringing out the "shine" it made the rest of the surfaces look somewhat "rustic?"! So I gave it a shot.
I do appreciate all of your encouragement - compared to the other superb engines currently being built - this is playschool stuff.
Anyway - talking of castings - this is what is provided for the cyl. casting :-

To me it didn't look too bad, having checked its dimensions with those on the plan, and for "trueness" across the various faces. The only real problem side was the base, where it was partly canted from true to the valve port face - marked in felt tip.
Following SB's suggested method, I decided to start with the port face side of the casting as "datum" and set up for machining the base in my milling vice with the datum side against the fixed jaw, and "squeezy" packing against the cyl. body.
Once done, turned around to lightly machine the port face and check the results with a square.
The casting bore seemed reasonably "linear" throughout, and went ahead and plugged the hole at each end with some turned down wooden dowelling (a length sawn off the end of a garden hoe handle - I expect to get "sanctioned" in due course when my wife finds out!).
After a quick facing swipe across the disc sander, I could then mark out the bore faces for the true centre using a height gauge on my Tesco face plate and popped the centre mark.
I didn't bother with any photos of all of the above as its pretty mundane stuff, but here's the result :-

The casting was then set up in a Keats angle plate on the lathe faceplate, using the two finished faces to locate in the larger angle, and some scrap steel packing on the round cyl. body.
It was set to turn true with the usual wiggler from the tailstock and clock :-

Subsequent to this shot, I bolted a counterbalance weight to the face plate opposite the mass of the casting/Keats plate. The lathe would then turn reasonably steadily at about 750 rpm.
After skimming the first face it became apparent that there could be some small casting flaws hidden in the gun metal material. You can just see the "spots" dotted around the face here :-

They don't look to be too close to the bore itself and are very small - so I decided to press on.
The boring operation was quite straightforward and I didn't bother with photos of this.
Pilot drill through both bits of wood (there is quite a large central hole in my face plate), then drill out to 1/2" and start boring with a Glanze boring bar with the tip set on centre (it will quickly remove any residual wood from the plug). I used a bore gauge/micrometer to check the bore at various points down its length as it got close to size (1" bore required) and allowed for "spring" in the cuts.
Obviously, a better way of doing this would have been setting up the casting on the cross slide and boring between centres, but my Glanze bar is pretty rigid, and using very light cuts at the end, I am happy with the resut.
I left the bore a tiny fraction undersize, and finished it with some wet/dry paper and paraffin, wrapped around another length of Garden Hoe handle, turned slightly undersize and kept moving in and out as the casting rotated.
I faced the end (removing 1/2 of what I needed to overall to bring the cyl. to finished length) and cut the shoulder, reversed the casting in the Keats plate and did the other face/shoulder.
Following SB's advice, I checked the valve and base faces for squareness to each other and the end faces by bolting the cylinder to the cross slide, and interposing a 123 block between the valve face and the face plate (the latter having been ever so lightly skimmed) and checking for high spots/misalignement. (Sorry, the top slide toolpost spigot is in the way) :-

There was one slight high spot, and as recommended by SB, I fly cut the two faces ever so lightly from a cutter held in the face plate.
Here is the result :-

You can see the casting spots - enlearged here :-

There do not appear to be any imperfections running through into the bore, and I hope those around the rim side of the end faces will not be a problem, provided a good gasket is used between the end cover/cylinder at this point.
I shall start on the end covers tomorrow (provided I can hide the Garden Hoe again).
Best wishes
Peter