Thanks all for your replies. I am a little reluctant to start sanding, using clay etc, as not a professional and worried I will damage the paint, which is original and pretty good condition.
I looked into Goof Off, but it would seem I can only get the professional type here in Oz, which contains acetone, when it would seem I need the heavy duty stuff. (No acetone)
I also went to a paint shop, bloke there said to try Alcohol. Said get some Mentholated spirits (alcohol) and mix 50/50 with mineral turps. Said the alcohol may soften it and the turps stops it from evaporating so it can sit and soften the paint. This stuff is rock hard, who knows how many years it has been sitting on the bike.
I still believe splattering is from a roller. Would seem he painted some strips of timber near the bike, cause later it looks like he place the timber on the seat and not quite dry and you could see where the edges of the timber sat. It came off pretty easy with turps, but not the over spray, unless the two paints are different and happened on separate occasions.
Like to try Goof Off, googled it here in Australia , can't find the non acetone type, unless any Aussies seen it about?