Grande Maurice

I need help people. I'm new to modelling but i've read lots, been to plenty of shows, and always been good at scenery... but i'm struggling with DCC electrics. I've acquired a 1.3m end to end TDM layout that needed upgrading and i'm extending it to 2.5m length with a new section (shown in the photos). It's 00 gauge Peco code 100 track with electro-frog points. I've acquired a NCE Powercab but will control the points off switches (more fun). I've laid and wired the track to a new DCC bus (thicker cable). Before i connect up the points wiring i want to check the track runs ok so i connected it up to the Powercab. However it's shorting out. Is this because i've not connected up with electro-frog cables (which are hanging loose underneath) or at this stage are they irrelevant? I've checked the layout, vacuumed it thoroughly and cannot see what the issue is. I've used track insulators as Peco advise on all points. I've attached pictures of the layout. The right hand rail (as you look at the photo) is always 'positive', the left always 'negative'. I've also shown a close-up of the Y-point so you can see how i wired the frog wire (this is where Peco advised me to). I'm concerned i may have made a fundamental error somewhere.....  If what i've done looks and sounds ok then how do i find the cause of the short? Any help really appreciated.