Update:
I have a HIVE thermostat, so turned the heating on from work at 4pm. Got home about 6pm, problem rads still showing top middle tems of 48 lounge and 53 bedroom respectively. Inlet pipe red hot (well 65 anyway with flow from boiler showing 70 using IR thermometer so prob slightly higher in real life). Hot rad close to boiler showing 65 degrees and rest "felt" good a hot. So no miracles occuring over night obvioulsy.
By the time kids were fed and watered it was 6:45 and so quickly decided to try the HIT IT & WAGGLE IT SOLUTI0NS ..............
1. TAPPED PIN HEAD WITH HAMMER 3 TIMES;
2. Tapped side of valve a couple of times;
3. grabbed pin with needle nose pliers and pushed up then pulled down on ping and, finally
4. gave the pin a waggle side to side 12-6 then 3 - 9 oclock before pressing up/down again;
For good measure also tapped lockshield valves and screwed closed, fully opened again........
No IMMEDIATE rocketing of system temperature, damn it (I thought -by now lounge up to 50 and bedroom up to 55 as 45 min has elaspsed with heating on full blast as set room thermostat to 25 at 4pm). Went and bathed kids, 20-min later front room and bedroom pretty much the same +couple of degrees more if that. Remembered over the weekend father in law suggested I set heating pump to no2 speed. Thought stuff it, give the system a blast and so quickly turned pump back to no3 (max speed) - which is actually what Vokera technical help guy told me to set the pump at today, without justifying his statement, when I explained the hassle I was having. Net result, within 10-min bedroom rad up to 60-degrees and front room up to 55degrees.
Has this solved it? - Not convinced to be honest and will need to wait until tomorrow AM to see as remember the rads will eventually warm up if you leave the heating on long enough (like 4 hours) and by this time the system had been blasting on full heat for 3 1/2 hours!
So sorry, but no conclusive proof of system solution (yet). One downside is at Pump setting 3 I now have 10-degree temp difference at boiler between flow and return. Not surpirsing as 8 of the downstairs rads are throttled right down to try and push heat to the rads at end of line. One thing I am certain of is turning the pump to #3 sped up the rate of heating of the problem rads (duh).
Once I solve the problem rads slow heating rate and get them to max temp same as the rest, (60-degrees within 30-min is my target for "success" based on his brothers bedroom radiator performing like this) I will balance the system again to try and get 20-degree temp drop at the boiler (55 return on 75 flow temp) as apparently this increases chances of condensing function working......
Will update tomorrow when Ive seen performance from a cold start.