I have found using a soda blaster saves the hard chrome, but scours-out the rust in the pits.
Realistically, the only long-term fix is probably going to be to replace the downtubes. Try Forking by Frank for replacements. Their online site isn't very useful, you'll probably have to mail them or call them. One thing, cheaper than OEM parts! I think they probably will run you about $400, shipped, but contact them for all pricing/shipping. I've bought from them before.
A couple stop-gaps, I still recommend starting with the soda blasting. Harbor Freight has a cheap set-up. You use bicarbonate of soda, probably the cheapest place to get it for most people will be a pool supply store. Costco also has bags of it which are smaller.
Some people swear-by rubbing balled aluminum foil into the pits, to deposit aluminum into them. I have never tried that.
Something else you could do is to try JB Weld in the pits and using fine sandpaper in higher # grades progressively to try to fill-in the pits, and to smooth them out.
I have used with success some coarse sponge-like material that tilesetters use to remove residue after grouting, Scotchbrite would be a similar product.
I see after posting you're in the UK, Classic Bike Guide probably has some ads in them for downtubes manufacturers.
I would NOT recommend using any-type of steel or even a brass wire wheel to try-to scour the pits. I think you will just make the edges of the pits more-rough. That will eat your seals in short-order should you replace them.
On a bike I once tried a hand-held fine dressing stone, to see if that worked, but I didn't replace the seals. It did seem to take some of the 'edge' off the pits, but once a fork seal is torn, it can and will continue to leak.
A trick the motocross boys use, is to cut a piece out of a plastic milk bottle, and push it below the lip of the seal to try and hook and remove any dirt/debris below the lip of the seal. The plastic should resemble the letter "J." You can buy a tool like that pretty-cheaply, or try making your own.
