I changed the thermostat and water pump on a discovery to 2001 and it's overheats and it was possible that when I put in the housing for the thermostat that I put the wrong hose by the wrong one and just need to switch the two hoses around as it doesn't seem to lose any fluid but it overheats and I don't have any heat
It is an external thermostat first time I've ever dealt with one

 

I would start with getting yourself a copy of the RAVE manual. It will have the info for your hoses, coolant flow, etc. @WaltNYC has a link in his signature for download. External thermostat - I'm assuming your running a stock one? I would recommend you change that to an @Extinct inline setup. Regardless of thermostat your running, bleeding the system is crucial. How great of a job did you do on bleeding the system? Sometimes it takes multiple tries to bleed all the air out. I'd do a search on bleed procedure.

 

There are dozens of threads on this forum relating to thermostats and bleeding the cooling system. Take a little time and read them. You need to get familiar with the layout of the D2 cooling pipes and the bizarre thermostat setup before you start swapping things around.
And once you've done some reading, you'll realise that you should switch to the inline thermostat as soon as possible.