It is possible that there could be some self siphonage happening. Try pouring a jug of water slowly in to the pan and see what level this settles out at. If this level is higher than you get after a flush, then there could be a problem with the configuration of your soil pipe and ventilation.
Just phoned supplier and they said that the toilet does have a low level that's just how theu come now. The cistern not filling properly is because of a limescale filter which I have been advised to remove as up here in scotland we have clean enough water not to need it.
If there is no limescale to filter, the filter could stay in place without causing a problem. It may cause the cistern to fill a little slower but it won't be the reason that the cistern only fills half way.
There sould be adjustment on the float height and a mark on the inside of the cistern to set the max level to. Modern toilets are designed for a maximum flush volume of 6 litres so will almost certainly contain less water than your old toilet.