Finally booked my flights after days of hassles and stress on the internet. Had problems with my profile on the opodo site - each time I found a flight the site lost the details as soon as I tried to pay. Eventually got round the problem by creating a new profile.
Despite this I'd recommend Opodo - it has a far wider choice of available flights to and from Mexico than the other sites (eg expedia, travelocity etc) - especially if you plan to arrive and leave from different cities. The secret is to force the search engine to find flights it initially claims don't exist.
So if you do a multistop trip and it claims there are no available flights then on the rejection page that comes up force it to search for specific airlines (option at the bottom of the page - eg Air France) - suddenly it will turn up possibilities it and other sites normally ignore. You can also force it to find new flights by specifying which time of day you want to fly. It's worth fiddling around with as you can make considerable savings. Just watch out for the transfer time as it may suggest flights with too short a switch-around for you and your bags to make the connecting plane.
Have added to the links (again) which I reckon are becoming pretty comprehensive. As always any suggestions appreciated. I've been letting several of the linked sites know about it. Even managed to get a free 3 months subscription to travelers meet as a result. The site is intriguing - it aims to match up solo travellers looking for potential travel mates. Will be interesting to see if it becomes popular - I guess most travellers would prefer to make friends as they trek, on the other hand there are tons of people posting travel buddy searches on message boards so there's obviously a demand.
Meantime I'm preparing for the various immunisations - seems I need quite a few incl. Hepatitis A, Typhoid, Tetanus/Measles boosters and Malaria pills. I was also offered a 3 week expensive Rabies series of vaccines and Hep B vaccines which I'll probably do without.
I had no idea how to sort this out and had to call one of my relations who is a doctor for info. It turns out in the UK you should phone your GP (family doctor) and ask to speak to the travel nurse. S/he will then quiz you over the phone and then book you in for an appointment. It's worth doing this at least 6 weeks before the trip as it may take a fortnight to get an available slot and the jabs have to be given a month before travel. Some of the more exotic treatments may require a hospital travel clinic appointment.
There's some more details at the below link from the USA and I've also set up a new list of health links on the RHS.
Another addition to the links is the remarkable google print. Don't have your lonely planet to hand and the online version doesn't have the info your looking for? No worries - google have digitised every page and let you word search. It's amazing technology and very controversial within the book trade - but invaluable for tracking down the number of the hostel/tour you're looking for. You need to register for gmail to get full access but anyone can try it out at:
