I spoke with Giant today about my front wheel & the lack of integrity in its manufacturer. They asked me to take the wheel to a Giant stockist in Bexley which I did.
The chap there confirmed what the mechanic told me on Sunday. The rimmed wheels aren't up to the job. It only had 24 spokes on the wheel & being my weight it doesn't deal with the weight distribution well & this causes the spokes to flex, which in turn loosen the nipples & this is why they come loose.
So another expensive trip to the cycle shop & I have just spent £200 on two new wheels which will be in on Thursday.