I'm morally certain the blade bolt is the same size as that on my ancient Elektra Beckum 300.
Applying lateral thinking to this, mitre saws are probably still made in huge quantities using that size bolt, as the arbour sizes were standardised a long time ago and haven't changed for decades.
You're able to get spares for all the major brands easily, as those blade-clamping bolts are particularly vulnerable to numpties attacking them with gusto, either turning them the wrong way (in which case they'll snap), or applying either Mole grips or a poorly-fitting adjustable spanner, in which case they round off the corners of the head.
Try Miles Tools in Yeovil, or powertoolspares.com (Southampton, I think). Have a look at DeWalt spares particularly (700-series saws, especially the 701, which is basically the old Elu design). It might be M6 tho. All those sites have diagrams on-line. You want a saw with a simlar sized blade on the same arbour diameter.
Also see the Elektra Beckum KGS 300 series (I have one). All that series use interchangeable parts, as did the Axminster induction-motored mitre saw (white-red livery, that they sold up until a few years ago - an EB copy).
That is almost definitely M8 LH, and the correct EB bolt has a wide flange on it, so no washer is needed. I think it's longer than 20mm though, so you might need to cut it down if it bottoms out (if it doesn't and it fits, leave well alone!). For ref, the KGS 300 (and its derivatives) has a stonking 250mm blade, but that's because the motor is quite wide and would block a smaller diameter blade, so it's not as macho as you might think. It does give a really clean cut though, in part because of the diameter.
Tips:
1. The blade bolts are probably always going to be left-hand thread on mitre saws, because those will self-tighten as the saw is used, for safety. This means you do NOT need to over-tighten them when you fit the blade, and that's also important to prevent the saw plate being distorted as it heats up in extended use.
So pinch them up, but gently - they're not cylinder head bolts!
2. Small table saws usually run with the opposite blade rotation to mitre saws (WRT the shaft), and so use right-hand-thread bolts instead. You can remember which handing it will be with this equation:
shaft on the right + cutting below + cutting away from you = LEFT-hand thread.
Change ONE of those conditions and it will be right-handed, change TWO it will stay left-handed, and change all three and you're back to right-handed again.
3. When you find a suitable substitute (e.g. Electra Beckum), buy two bolts at the same time. It's the same principle as the umbrella--a device you leave at home when you want it to rain--if you own a spare you'll never need it. At least that's worked for me so far.
. . .
I'm writing this in my dressing-gown at the breakfast table (would be quite anti-social, but I'm on my own this morning). When I'm tidy, I'll go out to the workshop, check the bolt size on the KGS300 and report back.
Later...
E.