Hi Karel,
The Orkneys are lovely, you'll have a good time with your friends. It can get windy in the winter but in May the islands can be really nice. The roads are quiet and a pleasure to drive / ride on. The islands are full of history, particularly naval history because of Scapa Flow.
You could try here for ferry info -
http://www.jogferry.co.uk/times.htmLooks like only 40 minutes from John O'Groats to Burwick. Burwick is on the south of the island of South Ronaldsay. This would be ideal for you. You could ride over the Churchill Barriers north onto Burray then Glimps Holm then Lamb Holm then onto the mailand. When you ride over the Churchill Barriers you can still see some blockship remains.
The place you have to see is Stromness museum, which is on the mailand -
http://www.orkneycommunities.co.uk/STROMNESSMUSEUM/Remains of one of the U 47 torpedoes are on display at Stromness. There are also artifacts from the scuttling of the High Seas Fleet.
Another place to see is the Scapa Museum on the island of Hoy -
http://www.hoyorkney.com/VisitHoy/scapa_flow_museum.htmlThere are some large artifacts of the High Seas Fleet here. The ferry is from Houton on the mainland to Lyness on the island of Hoy.
Simon, glad you liked the book. You are right about the schematic, I'd never really looked at it before. But it has what looks like Turm IV, which I don't recall seeing on any VIIB!
Cheers,
Dougie