Glad rag,
if you're a top rate taxpayer and you make any extra income, lets say a high street bank savings account then you must submit. i'm clearly no expert but thats how i understand it.
Abbotyob: forgot to mention that if this does affect you, just submit a paper one by 30 Jan with tax calculated by yourself. Or, explain the situation to Ty Glas and they will waive the self calculation bit and do the sums for you.