TALK about the Mother of All Excuses. We bumped into banker Paul Giles yesterday and were struck by the tell-tale bags under his eyes. What had he been up to, we asked. ''Blame it on my clients,'' fretted the Creditanstalt private banking VP, ''most of them have taken off for the summer to exotic parts of the world and they keep calling me up at all hours to ask how things are on the financial front.'' Pausing to rub his bleary eyes, Paul went on: ''Life is so much sweeter when they are in town operating in the same time zone. Last night alone I was awoken by clients calling from Mexico, North America and Canada. And one of them, would you believe, was more interested in talking about the weather than her investments - at 3 am.'' Of course, banker-cum-socialite Paul got no sympathy from us. We reminded him that money never sleeps - so why should bankers. Agreed folks?