[jezza944]

Just an update. All finished and good. Twist method with idler off is the way from a P guru I phoned to be sure.
One common fault when doing belts is overtightening the balance belt, hence the whine. The other fault is when tensioning, the tensioner can be rotated either way. From memory, cambelt is anti-clockwise and balance belt is clockwise. If the balance belt is tensioned anticlock, you can't get clearance of belt on idler, causing whine.
I also tensioned the balance belt with motor running till whine stopped, and checked and it was a neat 90 deg. And no whine at all!