Just a quick idea to get it up n running. Agreed B & Q are just a source for d.i.y ers but at times its came up trumps. We've an engineering suppliers locally where high tensile steel bolts or screwed rod and the such likes are available. There are many companies out there to buy grub screws probable better now a days as time and my tool box is not as full as it use to be. Best to get the store bought grubs.

You could try HUCO couplings as a replacement making sure there is a bed on the shaft for the grub screw to park into.

HI Seaspray
Yes you can use your method. I assume you are using stainless steel rod that you have threaded to the correct thread. However a slotted end will not allow you to achieve the correct tension on the shaft.
For high powered model boats stainless steel grub screws are the best. They are relatively cheap and will not fail if correctly installed.
B&Q may be good for general household hardware but do not stock suitable screws to use as grub screws.

if I am stuck I use some screwed rod of the same threads cut to length and put a slot in one end for using a screwdriver to tighten up. if I think its iffy about coming loose I use Locktight on the threads. I cut the slot with a Dremel using the very thin cutting blades. Do use safety glasses if you do use this as I've had them break off hitting me in the face

As an alternative to screwed rod is a trip to B & Q and buy some screws of the right thread and alter them.

Thanks for that Dave

Mine turned out to be M3 size and 3mm long which I managed to find along with some suitable Allen keys on e-bay.
In case anyone else is interested they are items :
180572239387 - M3 x 3 A2 Stainless Steel GRUB SCREWS (30)
and 400163232691 - 1.5mm ALLEN KEY (10)
Regards
Chris

HI Chris
Glad to hear you put your RIB on the lake. Grub screws are not the easiest to source. I have a box full of old couplings and many grub screws of varying sizes and threads but inevitably the one I require is different. I usually end up fitting a new collet or drilling out and re-tapping to a size I have to hand. Your local model shop may hold stocks and model racing cars used to use lots of them, so your best bet is to visit with your collet to see if you can find the correct size. Just a final thought - when you say rounded I am assuming a hex socket - in which case do you have the correct key. Both metric and Imperial are common and using the wrong one can round the hex. At my modelling club we all have boxes of keys and usually manage to find the correct one between us.
Hope you find what you need.
Dave