Unfortunately the 6 hours layover is upon our return, which is too short for a day room as we probably won't clear baggage reclaim customs until 3pm at the earliest, and that's if our flights isn't delayed and so that's why I was thinking of booking a lounge.

We have a 13 hour layover going out and are planning to use a day room for that.

I see you have answered my other question on the other thread, and my logic behind that was originally when we booked the Mercure they stated we could take our hold luggage up to our room. However they have since emailed to say that an EU law change means we can no longer do this which leaves an off airport hotel as our only choice (at a reasonable price).