Water leak from the heater ?

Wotcha' all, First off the motor is a 2002 Disco TD5 ES auto, here's the deal. Last winter I was topping up the water + anti freeze (approx 1 litre a week) all the time but once the spring came along and the heater was switched off there was no problem so I did bugger all about it. Air con on, again not a problem but now winter is fast approaching and I need to sort it. I've taken the plastic covers/ protector off from the engine and the hoses seem to be clear of residue so what do you reckon, has the heater had it? Also, I did have water leaking into the cab last winter. What is the process of elimination before I take out the facia to get to the heater?

any ideas or advice before I start?

Cheers
Al