Hurray!! Located and received some plan scans for the "JR More" tug from an ex South African now living in the UK! Delighted! Apart from a few misaligned scans that loose some pretty important info for the stern, I have a lot to work from.
The "JR" was built by Ferguson Brothers, Glasgow in 1961. 176.3ft LOA, displacement 1654.94 tonnes. She was the last oil fired steam tug in service in any South African port. Decommissioned in 1982. Now a poorly maintained (no money!) exhibit in the Durban Maritime Museum.

Thanks for info Doug. Unfortunately several of the websites get error messages or have dropped access to the address.
I think the Sturrock plans route seems to be the way to go (a pricey plan though for our horrendous currency exchange rate!) and was the source of the Ralph Wilmot model. Looking at some of the pics of the rudder layout when she was on the hard pending being put into the Maritime Museum, she had twin props and twin rudders...Sturrock didn't have twin rudders so obviously was built to the Sturrock plan. I have sent off a few emails and await any further guidance/info before I make any decisions.
Thanks Glyn re Durban Club. I have mailed a member too.

Here's the link to Ralph's posts on this site about the JR More.
https://model-boats.com/tags/durban-museum

No posts from him since Oct 2017, hope he's OK 😲
Doug 😎
Photos of original here
https://www.google.com/search?q=J.R.+More&client=firefox-b&tbm=isch&tbs=rimg:CWcFnttwT1OHIjjKd6y8Tddx_1fWOY4lRcepxXfNjo27kM-UWjCUs7Ux0DnhjxM_1z4xyzuA2x65x0yprrhupZzgOqoioSCcp3rLxN13H9EeWItg5WPC1DKhIJ9Y5jiVFx6nERjGHCo07FtsAqEgld82OjbuQz5RHjticXccPubioSCRaMJSztTHQOEYxhwqNOxbbAKhIJeGPEz_1PjHLMRhmGLuHbVyb0qEgm4DbHrnHTKmhH1L6dnGzndiCoSCeuG6lnOA6qiESm3jju0dC3H&tbo=u&sa=X&ved=2ahUKEwi-raK2zNvfAhXQI1AKHdMHDkgQ9C96BAgBEBg&biw=1920&bih=955&dpr=1

Hi Tertius,
Looks like the closest you're gonna get is one of her sister ships; the FC Sturrock.
https://www.cornwallmodelboats.co.uk/acatalog/Model-Marine-Plane-Tugs.html#SID=2608
There was a build article of using these plans to build the JR More in Model Boats in 2012.
https://www.modelboats.co.uk/albums/member_photo.asp?a=16223&p=327002
Another possible source
https://www.sarikhobbies.com/product/f-c-sturrock-plan-mm1480-tug-plan/

The guy who built it (Ralph Wilmot) also described it on this site back then.
Cheers, Doug 😎

Hi again! Have you tried contacting the Durban Model Boat Club? On their Facebook page they appear to have at least one model of an old SAR&H steam tug (not sure which one). They'd probably be able to assist with sourcing plans. Regards, Glyn

As I have made no progress on finding any drawings of the De Mist naval harbour tug ex Simonstown, and as kindly advised by Glyn as having been built by Dorbyl in Durban with VS propulsion, I am now switching my attention to the old steam tugs of the the late 50's and 60's...particularly the JR More (built 1961) that is currently decaying in the Durban Maritime Museum. Again I am on the hunt for drawings that must be more easily available judging by the number of models build of this vessel. She had twin props (rather than Voith Schneider or Schottel) which is something that I can handle!
Any help or advice gratefully accepted.

Hi Glynn
Thanks for your response. I was hoping that the dreaded words "Voith Schneider" would not come up as I have no way to engineer or buy a VSP (even looked in "Graupner" site but no info on buying units and I think it will break the pensioners bank anyway!
This is my first contemplated tug build so would be interested if the model tug Guys would perhaps advise what they do when the dreaded 2 words come up...do they cheat a little below the surface and substitute other propulsion sources when a VSP would be "true scale"?
Glynn, what Dorbyl built SA tugs do you have knowledge off that are not VSP equipped? I think the other older Simonstown Naval tug is the "de Nuys" but I presume she will also be Voith Schneider Propeller driven.
Regards and Happy New Year to you and all on MB's.

Hi Joburg-sailor! De Mist was built by Dorbyl, Durban, in 1978. Powered by two 8-cylinder Mirrlees Blackstone diesel engines with twin Voith-Schneider propulsion units. No idea if plans are available of this specific tug, but I do have some of similar vessels that were built by the same company for SAR&H in 1980 that are still in service (but probably nearing their service lives by now). Regards, Glyn