An impact screwdriver often helped me.
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You beat me to it - great tool. £10 on Amazon or £17.99 from Screwfix.
Here’s a lovely clip of one in action. Need to click on re-direct to YouTube to watch.
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Thanks, unfortunately I haven’t seen an impact driver that comes with a large and quite thick flat bit to suit
As one who restores ancient motorcycles for a hobby I endorse the impact screwdriver option. Re the bit - you can get large sets of bits quite cheaply which will swap into the impact driver, one of which will surely fit. They can also be used with rechargeable drill/screwdrivers.
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Also, do as you would when opening a bottle of Champagne.
Hold the screw still and slowly rotate the oven.
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Thanks guys, I’ll source an impact driver and a large bit to suit and take it from there, failing that it’s drill time!
If you can get to Cheddar car boot sale, I saw a new impact driver with bits there this morning for £9.50.
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I started my Motor Technicians apprenticeship in 1970 the Impact screwdriver was the go to tool for removing seized fixings as pictured, remember if you start to loosen it with the Impact screwdriver you can gently tighten and loosen to free it up.
WD40 was introduced at about this time we mainly used it to waterproof H/T leads etc as a release agent for seized fixings compared to Plus Gas it is worse than useless nothing came close to Plus Gas.
When you remove the seized bolt I would renew with either a brass or stainless steel type preferably with a Alan Head screw type.
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Be careful with hammering or impact driver if the screw is going into cast iron which is quite brittle and may fracture.
Another option for you would be to carefully drill off the screw head, retaining as much of the stump as possible. You could try to get your local blacksmith to weld a metal bar onto the remainder of the screw, and then apply a lot of heat ( oxy acetylene torch) to the piece that the screw goes into.
I have used this technique on stubborn vintage motorcycle bolts, and it works well.
Good luck
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Where do you live…Middle Earth? 
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Well done Neil, good guess, not ‘Middle Earth’ but the Scottish Borders, the beautiful centre of the universe.
In this area, believe it not, we do still have blacksmiths.
Blacksmith definition is
noun
- a person who makes and repairs things in iron by hand.
We call metal workers in this area ‘blacksmiths’ because they weld and repair metal things such as ploughs, tractors, motorcycles, cars and stoves.
I guess in more sophisticated cosmopolitan areas they are called by another name.
Good luck with your repair
Hugh
You could try a decent geared brace with a drill bit, that will give you much more torque to unlock it. Remember to grit your teeth whilst winding it in the right direction😜
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At the end of the day, this talk about WD40 or whatever the best/worst release agent for seized bolts is largely irrelevant. If a thread is seized enough that you’re waiting 24 hours for it to seep in, thinking of using impact driver or drilling the bolt out, a penetrating oil isn’t going to do any great miracle. It may work on lightly seized bolts to make it easier to remove and that’s it.
Heat is your best bet and I mean proper heat, localised. Heat from an acetylene torch, not a heat gun or general blow torch and with all that metal around it, it needs to be applied for quite a while.
And I don’t agree with the allen head being good for secure torquing in or out. It’s too small for the thread size. It statistically gives a good torque value, but not in practice when considering the bolt’s material. Allen heads are used on the best high tensile bolts in the world, this doesn’t mean they’re suitable for soft brass or stainless.
As ever, inevitably someone comes along in a thread and starts by spouting out their credentials to give them credibility. Well, I built my own racing car from scratch. It took 7 years of hard work and is a work of art. I’m also restoring a 1968 Mini Cooper and been into engineering for 40 years. Pathetic isn’t it, when someone spouts this?
If you want to learn about fasteners, read Carroll Smith’s series of books. Every proper engineer (particularly cars) would have already done so. Start with this one. I used to read this every night when I was a kid.
If your only knowledge about fasteners has been learned from this book, you are probably unaware you only know a tiny fraction of what there is to know.
Does the book differentiate the difference between a bolt and a screw. 
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Might aswell close the thread now then.
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But what would you use to open it again?
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In the days I drive Alfa Romeos, I had to carry a full toolkit with me, to effect running repairs and a friend discovered that I had a rubber mallet in the tool box, which ever after was known as the “Alfa mallet”.
You could open almost anything with an Alfa mallet!
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