Thanks for these suggestions.
Unfortunately, there is no button marked "Clear" on the remote control. I shall try the "4" button and report back. However, I have found a workaround of sorts.
What I did was put the Sony-recorded +RW disc into a computer and carried out a quick format using Nero, it takes around 1 minute. I then put the disc into the DVDR725H which then gave me two options, one of which was to erase the disc by hitting the record button on the remote control. This procedure was completed in around 1 minute and allows the Philips to record to the disc. Not very convenient if you happen not to have a computer with Nero handy but better than nothing.
I'll report back on Jethro's "4" suggestion.