Welcome Thomas
The Bentone BVP 300 is a realy old heatpump (like mid 1980 or something )
Bentone stopt making heatpumps when the Enertech group was founded and all heatpumps are now made by CTC.
CTC Swedish website
http://www.ctcvarme.se/web/Hem.aspxCTC English website
http://www.ctc-heating.comIf nobody on the forum can
you try contact CTC and see if the can
Now when the history is done lets try to solve your problem.
It can be more than one problem here.
First it sounds like your heatpump can't get rid of the heat it produce.
Do you have any ballvalve on the pipes going to the outdoor unit that looks like the one in this post?
http://www.varmepumpsforum.com/vpforum/index.php?topic=42535.msg410033#msg410033That is a filterball valve and needs to be cleaned on a yearly basis.
If you have one, clean it and see if that helps.
The other problem is far more serious
When starting your heatpump how does the outdoor unit looks like?
Is the ice layer bilding up on the entire surface or just on one side?
If it's only on one side or in spots that may indicate that you outdoor unit is low on gas.
If thats the problem then is better to buy a new system bacause Machines that old use the now forbidden R22 gas and can not be filled (retrofitted?)