Well Mac ............ no wonder you lost the will! Building Inspectors are usually only too pleased to let a Structural Engineer take responsibility. If your Engineer is confident in his calcs, I would be inclined to take it to the Chief Building Control Officer........... they have a complaints proceedure.
They're not likely to contact you, unless they have a purge on all "open" cases i.e. ones like yours that have not been completed. That's not definite ....... but most common.
The 4 years applies to Planning infringements. As far as I know, there is no "statute of limitations" on B Regs liability, I'm afraid.
B Regs liability passes to any new owner as I guess you know, but if you don't sell ...
"Too near a drain" ............... your drain or a public sewer?
Did you apply with "full Plans" or a Building Notice??