Hey Martin, My GS has done this for a few years now and I oddly a recent regas of the AC helped and boosted the passenger output from stone cold (even when max heat selected on that side) to luke warm.
If you research online you'll see that the heater servos can play up - mine appear to all be working okay and I'm loathed to rip into them again as some are a right pain to refit into the heater guides.
Issues are also reported with the heater matrix's which have a design that can easily clog on one side of the vehicle, the passenger side for right hand drives. The design has both the feed and return pipes on the same side (drivers) so coolant flow across the matrix isn't even and the far side (passenger) can clog up. I've flushed, soaked and back flushed my matrix numerous times and still find small black particles coming out, from where I can only speculate. After all this, the passenger heat output has improved a little but the drivers side is really good now! It's been a while since I did a flush, so I'll try and get another done when the weather improves.
Cabin temp sensors might be worth looking into if you use the Auto function but we don't and still have the issue. Maybe I should test Auto again and see if there's any improvement?
To really investigate/fix either area any further I'm looking at taking the dash out but only when the ambient temps increase. A new heater core/matrix can be sourced for just £46 from Rockauto + p&p, so whilst the dash is out I'm tempted to get one and replace mine anyway
Dave at Lex-Tek has seen a few older cars with clogged heater cores, he replaced one in an RX not long ago that felt much heavier than the replacement.
I will also be keen to hear if other folks have fixed this issue.
Good luck.