This is the core of the problem, ('scuse pun)
If your cooling system is sufficiently full of sludge and debris to clog up the heater in the first place, (and it does seem that Lagunas are very good at doing this), it doesn't take long for the cr*p in the rest of the system to 're-create' the problem.
(If you note my original post you'll have seen that I had the same issue as you.)
As your car has air-con, it'd be a big nasty job to remove the heater core itself - and probably not necessary.
1) Repeat the heater flushing process as per my post - I bet you'll be surprised just how much muck has found its way back in there!
2) If you have the time, (make the time?) to drop the upper and lower radiator hose connections and use the garden hose to forward and back-flush the Rad to stir up any muck and debris and wash it out. The bottom of the Rad is another great place for sludge and muck to collect.
3) Its probably worth running a bottle of Rad-Flush or something of that nature for a day or two prior to doing this as it'll help dislodge some of the well-adhered muck and make it easier to get out.
4) Re-flush the heater core at the end of the process just to be sure.
5) I know its a pain - but if like me it worked for a few days then stopped - the chances are that if you have two or three goes it WILL stay clear ... and whats the alternative?
Ours is still absolutely fine a year+ down the line.
PS: once it is OK - make sure the cooling system has the correct amount of anti-freeze going around it, otherwise you'll likely have water pump bearing failure a few weeks later - (AntiFreeze properties are used as a lubricant).
Best of Luck,
Let us know how you get on