Thanks for the shipping information. I finally used South Africa Airlines Cargo, on the Cape Town number that you provided. It was easier and cheaper to use them that any other shipping company (and the bike was in London in two days).
They charged 23.78 (£2.2) rands per kg; they didn't weighted the bike at all, believing the rather optimistic oficial weight given by KTM on the owners manual of 154 kg dry; on top of that they charged 220 (£20.2) rands for dangerous goods and 184 (£17.1) rands for handling charges. The total came to 3662.12 rands (about £350).
Because I had a Carnet for the bike the paperwork was minimal, they only checked that the frame number on the bike matched that of the carnet and them stamped it, nothing to be paid.
The bike wasn't crated as they say it was not necessary; only the petrol have to be drained and the battery disconected.
When I when to collect the bike at the London cargo office in Heathrow, I have to have it cleared through customs, that took half an hour (EU registered vehicles do not pay duty), then back to the cargo office where I paid £25 for handling at this end and £8 pounds for the customs entry document; collect the bike, fill it with petrol on the petrol station opposite the cargo office (how convinient!) and ride into London.
Once again thanks a lot!