Soundproofing underlay?

[Symptoms]

by the way does the sika system need plywood on top before the floor goes on?

No, it doesn't. The Silent Layer Mat is sandwiched between existing floor & the new. It relies on permanent 'elastic' bonding.

Factor in cost of additional ply, time taken to cut & fix (with screws!), buying other acoustic products to lay on top so building-up an untested floor versus a pucker 'out-the-box' sound insulating system - no contest.