More feedback.
I flew back from BCN last night and used the new 'service'. It's overall fine but not as seamless as it used to be. Not sure if i will use it going forward.
I went to bus stop 25 (at T5) and the Avis Shuttle came within 2 minutes - not bad.
The drive from T5 to the Avis parking took 8 minutes. The driver told me that there is a shuttle appx every 10 minutes.

As i wasn't sure about the driving time, i only booked the car when the shuttle arrived at T5 - which is not great considering that there were only 5 cars available.
On a separate note, they removed the Heathrow fees for the month of April, but the hourly rental rate has gone up from £14 to £16 - not great.