anyone got a how to guide??

No how to's as far as I know on here, but this may help - http://workshop-manuals.com/skoda/octavia-mk1/heating_air_conditioning/heating_air_conditioning/heating/removing_and_installing_heater_unit/

 

To be honest it is not going to be a nice job, you may want to consider an independent who has done this before, as it is almost a full interior strip at the front to get access.

 

Have you tried a leak stopper at all?, or is it chucking out large quantities of water?.

its blocked :( and blew the headgasket