I stumbled across this cheap as chips tandem in John Lewis today. It's only £105 from Kiddicare and I found it comparatively light and easy to steer but sturdy nevertheless.
But... the back seat is no good for a newborn. The harness is fixed at the extremes of the seat so looks like a baby would be flopping about in it. There's also a gap at the end of the seat which can't be closed unlike some tandems. I'd be scared the baby would slip forward.
Anyway, I thought about using a P&T cocoon in the back. Has anyone done this on another pushchair with success?