Rhodie Renewal

Hi - I would be very grateful for some advice.

I have a bed 18m long by 2m wide which is planted with dwarf azalea and rhododendrons.  Unfortunately, the bed wasn't prepared properly and after much deliberation I've decided to lift the young, small plants, scrape away the old straw mulch, weed and weedkill the whole bed, add plenty ericaceous topsoil and some good weed membrane then replant everything.  Can anyone give me an idea of the best order to lay it all down?  Membrane above or below new topsoil?  Plan is to eventually top the whole lot off with bark chips that I have 3 spare tonnes of (don't ask!).  Thanks for any help!