Hi again - PHW were quick to answer, most importantly, they refer to this guidance document bit.ly/ZscrSL (page 216-217)
You can see their answers on their Twitter page (everything that is addressed to @justthevax, scroll down to the first tweet above the one with the big green + sign and then read from the bottom up). I hope this helps!
They do stress to follow your GP's advice at all time (which sounds reasonable). Make an appointment, talk to your GP and take it from there. One possible solution would be to vaccinate your little one and take a titer three months later and if he is immune to measles then not vaccinate until 3 years 4 months, if not, give the next MMR then... hope your GP will find time to discuss this with you!