Just to close this thread off, and provide some help for others .... I found a hotel in Lat Krabang, called The Phoenix. The circa 800 bht rooms were clean and basic but had reasonable reviews. The girl on reception spoke great English and assured me a very safe spot for the bike, at no charge.
There was no insistence I came back another night, and there was a free shuttle both ways to the airport.
One month didn't seem any sort of a problem and I felt longer would have been OK.
The bike (and my riding gear I left with it) was untouched on my return.
The hotel is down a quiet Soi, and before storing, the shuttle driver even cleaned & waxed the bike for me (gave him a 100 bht tip). They gave me a dry porch way entrance to some other rooms in a property across the Soi, which was clean and behind a locked gate at night. The staff also used it for their scooters.
Hotel address is :
Phoenix Hotel
88 Soi 7
Lat Krabang District 10520
+66 27371446
GPS - N13.72321 E100.75388
Very happy to recommend this to any others in a similar situation, and you can book the hotel through booking.com if you prefer.
Russ.