I now realize what a bargain I turned down recently. I was offered an Olympus OM Zuiko 1000mm f/11 at the camera repair shop when I had my OM-10 serviced. Only £1000 including original case and key.
I already have the adapter...
The Samyang 650-1300 is a lot cheaper and much more versatile
€ 329,00 in the Netherlands
The Opteka branded version of the Samyang come with a 2x teleconvertor, for those occasions when 1300mm is just, well, too short...
£247.51 inc. P+p