Defo matrix leaking by the sounds of it, unless you are unlucky and have a leak on the engine elsewhere, and a water leak into the car.
I always put in newdue to the hassle of doing the job, around £35 last time I bought one, most of the rads are polish origin and aluminum, which is nothing like as good as a decent copper one, I made this mistake with a mk3 astra with a new matrix, a job I'm having to do again to put in a genuine one, at £90, but it's roasting hot, in fact best heater I've ever known in a car, even in this weather it would be on fan1(of 4) and the temp setting turned halfway down, at the mo it's fan 2 or 3 with heat fully up to get same heat.
tabetha