ok so iv set up my viv.
iv got my heatmat, ect ect sorted.
now im only getting a top temp of 17 with the mat outside the tank.
and a top temp of 20.5 with it in.
now its secure to the bottom of the viv with tape, newspaper and a layer of aspen.[inside the tank]
he cant get to it and hurt himself and waters at the other end of the 3ft viv.
iv got his hide on top of the heat mat. is this right?
if i take the snake out and keep the viv open the temp seems to rise to the temp i want about 25c, put him back in.. temp slowly, close viv drops back down to 20/21c
now i know he needs a temp of 23-28
how do i achieve this?!
iv got the thermostat propes at hot end and cold end, stuck to back wall of viv.
is this right? or should the propes be touching the bottom of the viv one on the heat mat one on the wood?
need some advice asap.
thankyou!
any other advice would be great.

very new to this.

you need to have the thermostat probe in touch with the mat. Your thermometer probe needs to be in the hide. A mat will only heat what it is directly in contact with, they do not heat the air.

To be pefectly honest, any vivarium housing a snake would be better off being heated with a ceramic heater on a pulse stat.