Hi Guys - I need some help

I got a 1998 V40 1.9D a couple of weeks ago and checked all fluid levels etc when I got it. Last week when driving at night the inside screen fogged up bad with an oiley film (now I know it was antifreeze)
Yesterday driving home from work (1 hour either way)the temp guage went high and the screen fogged up, I pulled into the first garage and topped up the radiator with approx 7 ltrs of water/antifreeze (was ok 2 week earlier)
Today coming home from work again same thing and same amount of water to top up. There is no external signs of large amounts of water leaks but was a littel puddle under the pedels (right hand side in UK).
I suspect the heater matrix has gone??? where the hell has all that water gone??

How do I get to the heater matrix as I cant find service manual??? Tell me it's not a tough job as its VERY cold outside

Thanks in advance for any words oif wisdom
Nugget