Thanks it explains the timescale difference in that I hadn't realised there were two very similar blue tartans - suspect the Laxey one was done both to simplify manufacture and also to avoid copyright - descriptions of early Manx dress talk about a plaid shawl etc but no specific design (I think most of the Scots tartans are 19th century inventions anyway).
On the more serious point I'd love to see the earliest date the 'Gunns' claim to use the 3-legs as its first recorded appearance is after the Scots takeover of the Island after the 'Battle' of Largs - Barney Young has traced the possible transfer of the 3-legs symbol (eventually dwindling to a single foot!) to Scandinavia - I wonder if the adoption by the Gunns was a deliberate and much later claim to descent from the Lord of the isles