Thanks for all your replies guys. This is the update. I have fully comprehensive travel insurance with Atlas who don't want to know because I booked through Trailfinders who have offered me a full refund. The travel insurance people say if I can get a refund they don't have to pay for alternative flights. They also don't offer any compensation for accomodation etc booked. Trailfinders said that no alternative flights were available and that if I wanted to go on the Air Namibia flights from Frankfurt I have to pay £500 which is the current difference in fare between the London flights and the ones from Germany. AND I would I have to pay for my own fare on another airline to get to Frankfurt. BUT the last poster is right. All other travel companies I've spoken to (including Dial a Flight who are horrified at my situation) have said Air Namibia has offered their passengers free transfers to Frankfurt on BA and free onward flights from there and they've transferred all their passengers in this way.
It seems that only Trailfinders have a clause in their contract which says they don't have this cover. I've spent all day on the phone to Trailfinders and they won't put me on the alternative flight and have now given me a refund which means I have absolutely no comeback with Air Namiba to try and put me on the Frankfurt flight.
WARNING Trailfinders say they are the 'travel experts' but at this moment in time they are not being very 'expert' with my situation. I can't believe that they are one of the most reputable travel companies in London (I have booked with them for years) and they are flatly refusing to do anything else to get me to Namibia.
I'm lost for words...