So the pier is now complete and I'm happy with the final result. Pretty much everything is to specification (with the exception of one minor niggle) and it's extremely sturdy, which is something I was concerned about initially considering how tall it is.
It passed its first test which was to slot the base over the bolts in the central pillar. I was a bit worried about this as we did have to force down the wooden template a little when we put that together and with steel you don't really get the same room to force things. Fortunately it fits perfectly.
200mm bolts will provide a lot of leeway if I ever need to adjust height for whatever reason. If I get another mount in future it shouldn't be difficult to get the top plate to the ideal height. With the setup of the nuts it's very secure.
I degreased it and gave it a couple of coats of paint for rust resistance.
I will need to sand the plates a bit as the paint has blocked the bolt holes. I'll probably gently sand the top plate as well just to get it nice and smooth/even for the mount to sit on.
I asked for a small countersink for the bolt holes on the base as I was concerned that the bolts in the central pillar might be too short and I wouldn't be able to screw the nuts down onto the base enough. Unfortunately I didn't think to specify that I needed the countersinks flat and you can see the consequences above. Regardless, I am still able to screw the nuts down pretty tight and get the pier sturdy enough. There is some available length on the bolts as well though (see image second from top) so I think I will still get some small metal plates to sit over the countersinks and just screw down on those anyway. On such a tall pier, the contact at the base is where the rubber meets the road so to speak so I really do want to get it as secure as possible. If it takes a bump then it's going to spoil any exposure happening at the time but there is no way around that. The real test is to see if it picks up any vibrations while in use though. I don't believe this should be possible anyway as the central pillar does not make any contact with the observatory structure at any point. Will be a bit anxious till this gets proven in practice though!
Anyway, there's nothing to stop me going ahead and putting the dome up now so that'll be my next update. I'm not sure when that'll happen, just waiting for a fine day without wind or rain and at the moment it seems we always get one or the other. Hopefully it should happen sometime within the next couple of weeks.