Basically my question is:  Survex? Therion? Compass?

Which choice to i go for?

Initially surveying to try to relate 2 smallish (<1mile) cave systems which are very close to each other and to various known sink holes that have been found in the dry riverbed above in order to asses the likely hood of various potential digs/boulder chokes being fruitful.

Secondly to produce some decent plan and elevation maps for other cavers.

I've had a play with Survex up to now and have got my head around it enough to get the basics for a simple survey. I've not yet had a serious try with Compass or Therion.

A bonus would be to be able to run the software on a Windows PC without having administrator rights to install it. Survex/Aven does allow me to do this - i just told it to install to a folder on my dekstop and ensure that cavern.exe is set to run in windows 95 compatibility mode. (So i can work on projects during my lunch breaks on my work pc where everything is locked down).

What does one package give me that another one doesn't?

With all due respect to Larry, Compass won't give you anything that Survex cannot, except for a prettier front end and if you are OK using Survex, then stick to it. Therion is a much more complex drawing tool, takes a lot more understanding and is (in my view at least) probably less suited to beginners. It is also quite straightforward to port Survex data to Therion in the future.

So start with Survex would be my advice.

I'd second that. Plenty of advice already in this forum on getting started with survex. 

(welcome to the site by the way)

You might also want to take a look at Tunnel (http://www.freesteel.co.uk/wiki/index.php/Tunnel) which works directly with your survex data and allows you to draw detailed plan surveys of your caves. If you already have a recent Java runtime installed when it should run on your locked down computer OK.

Thanks for all the advice which pretty much seems to mirror my initial assumptions. For actual survey work I was going down the compass/clinometer route but i've just 'won' a Distro A3 on ebay and contacted Beat Heeb about the upgrade boards - which are currently out of stock with no new ones currently being made - gutted