Carnet
The key document for an overland trip is a carnet de passage - this allows you to import your vehicle temporarily into countries outside the EU without having to pay import duties. The body in the UK which is responsible for issuing the carnet is the RAC - there is a wonderful guy there called Paul Gowen who knows all there is to know about carnets and is one of the most helpful people I have come across (see Links for contact details).
The cost of a carnet is based on a multple of your vehicle value depending on which countries you decide to travel to. Unfortunately, Egypt is the most expensive at 800% which meant, in our case, even having written down the vehicle value to its lowest book value, our carnet cost was £28,000. Fortunately, there are a number of ways you can cover this -
1. issue of bond to the RAC for the cost of the carnet (not particularly attractive option);
2. pay for an insurance policy, organised through the RAC. The premium works out at roughly 10% of your carnet cost, 5% of which is refunded to you once you return a completed carnet on your return; or
3. persuade your bank to issue a bank gurantee to the RAC to cover any import duty liabilities which may arise. This is by far the cheapest option but also probably the most difficult to organise.
We went for option 3.
Visas/passports
The only visa we have had to secure in advance is for Libya (which be warned, is costly!) - the others are either available at the border or are easier to obtain from embassies en route - see Country Info for visa info on each country.
Vehicle insurance
There are only really two companies that provide vehicle insurance for overland trips through Africa (for accidental damage, fire and theft only - not third party, see below) - Cambell Irvine and Locktons. Locktons came back with a quote which was almost half that of Cambell Irvine's and have been very easy to deal with (no need for a vehicle condition report for instance) and therefore we have gone with Locktons. It is a fairly pricey premium hence many overlanders take the risk and do without.
In many countries, third party insurance is required to be purchased at the borders. Once in Ethiopia, we plan to purchase a Comesa Yellow Card which covers third party liability and driver medical expenses in a number of the southern African countries including Burundi, Democratic Republic of Congo, Eritrea, Ethiopia, Kenya, Malawi, Rwanda, Uganda, Tanzania, Zambia and Zimbabwe. No insurance is required for Botswana, Namibia and SA.
Update 24th Jan 2010 - Comesa Yellow Card : We purchased our Comesa in Nairobi through APA Insurance in Westlands. In order to get the Yellow Card, you have to also purchase local insurance for at least the time period that the Comesa is required. This is quite expensive in Kenya so would recommend trying to get your Comesa in Addis where we have heard the local insurance is cheaper. In total, we paid KES9,600 (approx £80) for 6 months local insurance and 6 months Comesa (the Comesa element is a fixed price for 6 months of KES3600 - although I think they got their maths a bit wrong in our favour!)
Medical insurance
Mainly to cover medical expenses and repratriation costs if anything goes wrong - we found the best and most reasonably priced to be Navigator travel insurance world wide cover.
International Certificate of Motor Vehicle (or Carte Gris)
This is an international translation of your UK V5 certificate. According to many overlanders experience, although strictly a requirement, they were never asked for it in their entire trip. We have got it just in case and it is easily aviaiable through the RAC when you get your Carnet for a small fee.
Update 24th Jan 2010 - we have been asked for our Carte Gris at most borders in Northern Africa so a good job we got it in the end!
International Driving Permits
A must for driving though Africa - again, easily available through the RAC with the Carnet or at your local Post Office for a small fee.