No problems.
It's written in a way that the valid and sufficient reason is at the discretion of the local divisional returning officer, but essentially the reasons are that you were not able to be released from work that day, you were ill, or you were genuinely travelling. A whole range of things may be valid and sufficient reason for not voting.
The way that's uncovered is we write to every non-voter after the election and they write back to us and explain why they may not have voted.