I popped along to the Stay and Play session at Towcester Chlildren's Centre this morning http://www.southnorthantschildrenscentres.org.uk/index.php?page_id=59 I don't live in Towcester but made the trip especially to use their sensory room. It's great and DS loved it. It's a nice children's centre too as it's really spacious. We'll definitely be going back. And if you're registered in one Northants SureStart centre you can use them all without filling in the form.
Just thought I'd let you know as it was well worth the visit!