Update:
So after several days of Plusgas down the tube and tapping with my wooden mallet there was zero progress. Today I went to the Seatpostman in Chorley and we are... FREE AT LAST! John did an excellent job. He said it was a longish post. So even though I thought there couldn't be much post in the frame it was a 30 or 40 cm post. He said it was corroded all the way down to the bottom though I didn't see it I believe him. He explained the process by which the aluminium corrodes and expands is called 'exfoliation' and creates high pressure against the seat tube. So no amount of freezing would have freed it. It was a 27.2mm Shimano post - 'modest' in his words because the clamp has only one screw to adjust position on the rails. I said SJSCycles are selling their own branded Thorn 27.2 posts at £10 and I was thinking of getting one. He said better get a post with two screws fore and aft to position saddle on rails easily and precisely. So i've held off that for now.
I brought my saddle from my Harry Hall audax which is on a 27.2 post with me. It went in fine though we didn't tighten the clamp up at his workshop. After the 90 mile round trip I tried it at home. But there was a PROBLEM! The clamp male bolt wouldn't tighten sufficiently before reaching the end of the female bolt. It just went to a hard stop and the post, though tight enough not to slip down, would easily rotate with one hand. I did a quick first ride after several weeks of looking at the bike. Frame size seems fine even as a 'large' 56cm ( I normally ride a 21 1/2" /54cm), just needs new brake pads. So I phoned John thinking something major had gone wrong. He was very helpful and reassured me - rarely someone puts the wrong bolt in the seat clamp. And this could be the only explanation; seat tube, post and clamp were all fine. How the wrong bolt held the post in the first place I don't know. Maybe because there was corroded aluminium inside the tube walls producing a sort of friction fit? As it had seized it was academic anyway. He said it more often occurs when people use the wrong length bolt in chainrings on the chainset. They tighten and tighten an over-large bolt but get nowhere until they discover their error. I asked if, when he cleaned it out afterwards, he might have removed some of the tube walls. He said not significantly - the swelling is from the aluminium corrosion or exfoliation. He advised me to check the bolt shoulder length with my vernier gauge. I haven't done this yet - just measured with steel rule. Then i realised the Harry hall has a similar seat post clamp and top it out. it seems to be a match for diameter - 8mm and significantly shorter. I tried it in the Hewitt and it fits - I can now tighten the pinch-bolt up properly so the post doesn't slip or twist.
While John was working - it took about an hour - I visited the Paul Hewitt Cycle shop in Leyland; ten minutes drive away from Chorley. It's an old building with timbered beams across the ceiling and nice and cool on this hot day. It was empty apart from me. An older gentleman appeared and asked if i needed help. I said I'd just left my Hewitt bike with John up the road and thought it would be good to see where it came from. He said 'I am Paul Hewitt' ! We then chatted about the Cheviot model I'd bought. He showed me a cutting on the wall with a photo of three cyclists taken from a BBC doco shown sixteen years ago called 'On Hannibal's Trail'. They were each riding a Cheviot! I remembered watching the three Australians setting off from Carthaginia and following Hannibal's crossing of Southern Europe and the Alps - with elephants - to fight Rome in the second Punic war. Clips from the doco are still online:
https://www.bbc.co.uk/programmes/b00t6skb/clips
