Hi Paula
Early sign backs often / nearly always have a grey wash, with odd splashes of colour. Recent repros are usually uniform in colour black or grey often with a track of two lines across them.
If you look thru the old enamel sign forum you will find ' fantastic backs ' which will give you some idea. If you check the Useful tips and warnings etc forum you will see some dodgy signs with dodgy backs !!
Obviously your sign is not early, neither does it appear to be a recent repro. The bike would suggest a manufacture date of perhaps the 40's or 50's . The makers mark / serial no. is strange, not one personally I've seen before. If its enamel ( which it appears to be ) it should be quite heavy. If it only weighs light it would more likely be tin.
Talk of £500 , £1000, or even £1500 may have got you both worried and or excited ? Your ebay listing is fine, more than a few people will have their eye on it & don't worry it will find its correct price be it £50 .............£1500 or somewhere between !!
Lummox
PS Nice Tattoo !!