If you decide to go with aluminium, you might try 
http://www.doremetals.co.uk/aluminium/tube/ 
for a piece of ready made tube. They list much larger sizes than most companies. With cylindrical partners, you can use pairs of opposing wedges, so that you don't have to form a conical hole.

I wouldn't think it too difficult to finish the outside of a moulded cylinder. Much easier than finishing the hull of a boat made on a male mould, anyway! Let's say three layers of heavy woven rovings, alternating with three layers of heavy chopped strand mat ( similar to the traditional layup of GRP hulls). Let it cure, then grind back to remove the bumps, but not all the hollows. Then a layer of thinner glass cloth followed by sandable filler (glass microspheres).

Making the mast partners this way, out of plywood, is going to take an awful lot of plywood, and will be expensive unless you happen have a large pile of offcuts already. When the mast has to go through an angled surface like this, it makes sense to think about a cylinder, either of metal or GRP, with a flange welded/moulded onto it at an angle, to bolt to the deck.  

Here's an idea for you: your drawing is showing a mast of 180mm diameter, so the internal diameter of the partners will be of the order of 200 - 220mm diameter. I happen to have a 10 litre plastic bucket that could act as a mould of that diameter, for laying up a thick tapered tube using lots of woven rovings on top of a gel coat. You get a ready-made taper suitable for either wedges or poured polyurethane, which you wouldn't get easily if you got someone to weld up something from alloy or stainless steel. Getting a smoothly tapered hole through plywood layers takes time, too.

You'll have seen the photo of my mast step. I still like this method. Just make the tenon as large as it can be within the diameter of the mast.

I have been reading a lot of earlier posts from you all, including one on the ‘Bad Tack’ issue and am now beginning to understand things in better detail and the amazing depth to which the JRA site goes. It must be a bit frustrating having to go over the same ground again with newcomers like me. All this information on sail camber and performance is fascinating and very instructive.

I have now visited the boat and established that the mast will fit forward of the hatch very nicely, it's definiteley the best position for it.First I will have to remove the existing water tanks and modify them. I have drawn up, here , how I think the masts supports can be configured (not sure what to call them) I am assuming that the upper support has wedges to locate the mast firmly and I assume these would be perhaps 35mm wide and sanded to the mast curvature and fitted all round the mast tightly. How would these be held down, friction alone? Any comment on this would be welcome. I am assuming that the upper support is an inverted cone with a bit of play at the lower end , I guess this allows for a certain amount of misalignment when the mast is lowered in. Wedges then hold it accurately in place after the lower end is properly located. Please correct me if I am wrong. I'm sure there must be info on the site somewhere regarding this but at the moment I cannot find it.

Having spent so much time thinking “junk rig’ I was surprised that when I was on the boat the Bermudan setup looked like an awful lot of clobber in comparison, I can already see the clean simplicity of the JR mast in place. However, there are a few hurdles yet to surmount (apart from actually making it all) including the sale of  small trimaran I built to fund it, and getting my other half to accept yet more expenditure on a boat that has already had it’s fair share off funds!

Annie Hill wrote:

Richard Oates wrote:With regard to pubs, there’s only the Dalton left now!

You're joking.  That's tragic: there were three in the village when I was there, two Thwaites and a Boddingtons - although the Dalton Arms was my favourite - and one on the road to Lancaster, Boddingtons, - The Stork: it had a wrought-iron bird as its inn sign.   They did splendid fish and chips and my old friend Eric used to take me out for lunch there on occasion.

I've heard that the drink/driving laws are killing off British pubs, but am shocked to hear such clear evidence of it.  It ain't the country I remember.

Sorry to mislead you, the Stork is still there and thriving, it's the one North of the dock thats closed.

David Tyler wrote:

That does sound expensive for Weathermax, but they are the only source.

I was quoted $49 per metre from one source, bought mine for $20 per metre and, had I bought a full roll could have got it for $18 per metre (all plus GST).  $20 is about GBP11.56 at today's rate.  My supplier was Safety Components and I see that you have the same company in the UK.  They might well be worth contacting.  (I never asked Contender to quote me.)