Hesford resigned because he no longer wanted to be associated with disgraced Attorney General, Baroness Scotland.
A PSS is a minister's little helper in parliament. They don't get paid; they aren't members of the government. Hesford's connection with Baroness Scotland was rather tenuous - he wasn't her PPS, he worked for the Solicitor General who is the Attorney General's deputy in the government's Law Offices.
The only real point of taking a job as a PPS is to get your face out there. You get to rub shoulders with members of the government and just occasionally you may deputise for your minister - if none of the ranks of more junior ministers want the job on the day. The only point of being a PPS is to get seen and promoted to something more significant.
So one cheer for Hesford. He has given up the prospect of a glittering government career, which in fact in this government of the living dead he never had. On the plus side by resigning he has got his face on national TV - and I for one, had never previously heard of him. Clearly a man who can spot an opportunity.