coving - mitre problem

[nailedit!]

Hi - can anbody help please?
I am putting coving up in my daughters' bedroom, and despite carefully following (and double checking!)instructions from a reliable DIY suppliers website can't seem to get corners right.
There is always a gap at the corners. I am using paper covered polystyrene coving (lighter for us girls to put up!) 13cm across. The box is a standard 10cm one.
All the diagrams show the coving fitting neatly into the mitre box, but mine is proud of it. Am I getting the wrong angle because of this? Should I put a block into the mitre to ensure that the top edge (fits to wall) is at right angles to the mitre box base?

[acsimpson]

If you're coving block is too small for the coving then you will find it exceptionally difficult to cut the correct angles as the coving will never sit flat in the box.

I think the best advice would be to take the mitre block back (if it's still as new) and buy a bigger one, or buy a specific mitre cutting guide.

I recently fitted 100mm coving for the first time using an appropriately sized mitre block and other than wasting a foot or so cutting an external corner rather than an internal one everything went up like a dream.