Oversized valve guides "Help" open valve type head.

You can't just buy  an oversize guide and expect it to fit a worn hole. If the old guide measures up at around .627 (is that its biggest diameter?) it won't be round and straight and neither will the guide hole. First thing is to ream the hole in the head straight and round and in line with the seat, Then measure the hole and get/make a guide to suit. I've dealt with numerous cylinder heads where the guides have fallen out when removing the valve springs. All of them had to have custom guides made to suit. I would expect the guide hole bore once reamed straight will be around plus 20 thou or so.
The procedure outlined in many workshop manuals to change valve guides even if followed to the letter will result in ruined guide holes in the head. The hard carbon around the old guide will knacker the bore in the head even if you heat the head as directed. The hard carbon galls and tears lumps of alloy away with it. Even blast cleaning the old guides will not remove the carbon deposits that do all the damage. 
The way to remove guides and leave a perfect hole is to machine most of the guide away and just leave a thin shell of metal that comes out easy.   
Your 'engineer' should have told you all this, if he says he can't make new guides or ream the head to correct the worn guide bore find another engineer who can.
     

Can I ask?  The 18 will have a cast iron head.  So is galling of the head when removing guides the same issue as with alloy heads?

Also: typical stem clearances which modern engineers are likely to expect or recommend are (it seems) much closer than with prewar engines with cast iron heads which don't necessarily have rocker caps to make sure tops of valve stems are bathed in oil.  Any comments, please?

Modern installers seem to aim for super smooth guide bores.  This seems odd, since smooth bores won't retain oil - will they?  And are such smooth honed bores relevant with wider stem clearances?

There seem to be huge numbers of opinions on best (or even just 'satisfactory') practice.  And that's before discussing valve guide material.

Is Galling of the (iron) head the same issue as the alloy head? I don't know as i haven't done a guide job on an Iron head. That said if the old guide in this instance is a 'sliding fit' and has had oversize guides fitted in the past it tells me that the procedure used to change the guides previously damaged the holes in the head and has resulted in slack fitting guides. Guides should have as smooth a bore as possible and the stem to guide clearance should be as the manufacturer intended, I'll refine that statement to say the valve manufacturer. This is to promote heat transfer from the valve to the guide/head. Old machines such as these use cast iron guides, even modern machines use cast iron guides, Bronze guides (the correct grade of bronze) are for racing use, the heat transfer is better than cast iron but the wear rate is worse, In racing use where the engine is almost in a permanent state of being taken to bits and rebuilt this is not a problem. 

One of our Surrey branch members gave us a talk and film show  detailing the work he had to do to get his early 18 into good running order. there was much detail on valve guides . He had to research and buy a specific grade of cast iron and get the guides made . I expect he will be along soon with some valuable advice. I can testify that his 18  runs and sounds magnificantly.

Decent article in the singles compendium on VG replacement, page 183.

The age old problem of galling during removal used to be overcome by single knurling the guide to be fitted. The heated head and liquid nitrogen chilled guide ( in a thermos) gave it an easy ride into the head. There is always a way....

Should you choose to go oversize, you must also recut / replace the valve seat to maintain concentricity with the guide.  

Knurling a guide to get it to stay in the head is a classic bodge, done by people who can't work to the limits required in making new guides and are covering up their ineptitude.

Earlier this year I overhauled my 1927 Model 18 head and found that the valve guides from my 1955 ES2 has the same internal diameter for the valve stem and a larger outside diameter, in the region of 0.059".
This may provide your overside guide off the shelf from RGM.
I found that Racing Norton (Paul Norman) was very helpful with detailed knowledge of early Norton heads.

Any automotive engineers that do head work should be well capable of producing a valve guide themselves. 

It's worth trying Cox & Turner Engineering Ltd, they're based near Yeovil and are excellent with piston rings, head work etc.  They supplied a set of +0.60" for my 1927 Model 18 off the shelf last year!
Give them a call on 01935 826816 and see if they can help.
Their website is also interesting;   https://www.coxandturner.co.uk/ 
 

Just one bit of experience to note if you make your own valve guides in Colsibro. 
Finish the size of the guide's press fit last. 
Machine a mandrel in situ on your lathe a good fit to your guide bore and then machine the guide on this. 
Otherwise machine the bore with a boring bar not just a drill and ream. Any runout will leave you a world of pain re cutting the valve seat. 
Ideally you should hone the guide once fitted.