Hi Gav,
I don't make the Chairman as yet but it looks like a Gents.
Most nibs in kits are normally as far as i have found either 5mm or 6mm, if this is the size of a full size Gents its mora than Likely 6mm.
To get the nib out is easy. Just unscrew the carrier and nib from the front grip section.
Then holding the nib with your thumb over the top of the breather hole and your thumb facing the plastic or ebonite, section and your forefinger underneath.
The grip the section with your other hand.
Depending on the carrier most either pull out or you have to turn the nib slightly, then pull out.
Then if your new nib is the same size place it over the feed part (combs) and replace.
Keeping the distances exactly the same as the original nib was, ie equally placed either side of the combs, and the nib point, the same distance from the feed as it was etc.
Thats a very basic idea as i am not an expert but they are out there.
If the nib won't budge drop into a cup of hot bit not boiling water for a minute or so and it should come apart.
I have had to do the same thing to replace a nib, it came out ok but wouldn't go back in without a little help.
I can sell you a JoWo nib but not Bock, but for an Italic grind i can get them done easily but simply pass the cost of the grind on, but its not cheap at £40 plus the nib. I'm not allowed to charge less. But there are lost of variations, crisp, cursive etc etc fine, medium, broad, extra fine, extra extra fine.
I can't put you in touch with the chap directly but there are several people in the UK who grind nibs apparently but i only know the one.
A very good website is here
RichardsPens.com • Pens That Write Right!
A video that shows how to do better than my very poor description is here
How to replace a fountain pen nib - YouTube
Pelikan pens offer a nib grinding service for £30 plus i presume the price of their nib, but i don't think the carrier will fit a Chairman but the nib might. If it is the same shape, ie same base shape and width, same size across wings and same overall length. Its a bit trial and error sometimes. But i have jad customers for branded pens simply swop over the nib and use their own carrier.
Pelikan Fountain Pen Nibs | Pelikan Pens UK, the Online Pelikan Specialist and Fountain Pen Nib Shop
Hope this helps
PM me if i can help futher
Regards
Pete
