Just a quick update on this as it may be of use to somebody in the future. When I put the Sig back together it was still popping a BB out the ejection slide about once every ten shots. Everything was working as it should and the guide seemed fine. Then I noticed there was a tiny gap between the plate holding the cylinder on so tightened the screws again and found when I thought it was tight there was still about a half turn to bed it down fully. With the blowback it had been moving the cylinder slightly and misaligning the guide.
So thanks to all for the help once again.
Cheers.