Simple answer too the question:
(a) Is it worth installing a dedicated charging point, well it all depends, we installed a 7kWh Podpoint as we intend switching over to a full EV in the near future, at present our PHEV will only charge @ 3.6kWh, a full charge takes maximum of 3 hours.
(b) To charge up from the granny lead takes about 5 1/2 hours
( c) An extension lead need to be full unwound/uncoiled when drawing high loads, they do get warm and have been know to catch fire? we use a 2.5mm cable which I have spare.
( d) If I plug my granny lead directly into any mains socket the 3 pin plug get very slightly warm.
At the end of the day the choice is yours to make even with the grant you are looking at around £500, that a lot of Octopus Off peak 5 pence kWh, that 10,000 kWh
Unfortunately I have suffered a brain injury and occasionally I get confused and often say the wrong thing.