I rang Etihad UK as suggested. The Etihad rep was not able to get a printed / emailed receipt off the system with fare & tax details, although he was able to quote them to me.

He suggested that this was normal for Guest Redemptions that included a mix of taxes and cash (I paid a small cash amount to reduce the miles element of the fare).

As far as expense claims go, there is no way to get a full receipt. (But I have a M/C statement and will use that.)