have repeatadly retuned the recorder to obtain BBC1&BBC2 channels,they will remain operable until the recorder is put on standby,whereby next time the recorder is used these two channels are missing.Retuning again will restore them as above

raymond cleverley, September 2011

This is a known issue.

The Hitachi HDR081 is made by a company called Vestel. This fault applies to many of their old freeview recorders. Yours is a version of their T816 model.

Follow this link for details and the fix http://www.vestelpvr.futaura.co.uk/dso.php

To quote:
... these boxes are currently limited to a maximum of 99 channels.

The recommended solution for avoiding this is to perform an automatic channel scan and immediately manually delete any unwanted channels from the channel list, so that you have less than 100 channels.

T816 owners should also ensure that the "Scan encrypted" option in the setup/configuration menu is set to "No" which will ensure that most of the channels that cannot be viewed on non-TopUp TV boxes anyway will not be added to your channel list.

NS, November 2012