No, a fully working 200TDi radiator will happily cool a 200TDi engine in any weather, the heater is a seperate loop off the thermostat housing. The only differences a blocked heater matrix would make are that your heater wouldn't work and the engine would warm up a bit quicker.
Overheating of an otherwise un-modified engine* is a massive airlock (see Ralph's response), a blocked radiator, a leaking cooling system or the thermostat staying shut (test by dangling in hot water straight out of a boiling kettle to see if it opens) but in this last case the heater should be hot enough to strip paint!
So, could any of the other things be the problem?
* Tweaking the fuelling, running a different engine count as modified