Ok banging my head against the wall is not solving my problem so im asking the experts (YOU) so my wifes mk7 has a strange problem, starts fine 100% on the first start of the day now once its hot it may or may not start it does not try start it just clicks. I thought ah must be starter motor or battery, it had original one in so got the correct one, calcium one i believe and yet still the same, checked voltages of battery and seems good running and off. 

so it did it the other day going to local shops all i did was pop the bonnet and let it cool down after about 10 min it would then start no problems so prob not the starter motor, so i used a start pack and it would start no problem so basically my question is, is there something in the engine bay which reacts to heat and would prevent a start up?

many thanks in advance Ben

Could be the battery or earth leads. If they are faulty then they will be worse when hot. Check their impedance with a multimeter when cold and hot. 

Having said that; the same applies to other electrical parts.