Your sat in T5 and rather than speak to one of the 50+ Passenger Handling Staff on duty, you decided to ask on Flyertalk? Mind... blown! Go and ask, the suggested method is as follows...
"Excuse me, I'm on the delayed BA59 to Cape Town, can you tell me if its still planned to be the same Aircraft type? If not, can you have a look and see what seats are showing for us?"
How on earth anyone can get to Silver and not know when to be proactive and speak to staff during IRROPS is utterly beyond me.