Last time I did my 90-day report, the slip of paper in my passport was the new barcoded one and I was told that next time I reported I wouldn't need to complete the form, just hand in the passport.

My report is due this week but it's looking likely that it will be impossible for me to get over to Chaengwhattana in time due to work commitments, and I've left it too late to do one by post.

I will be leaving Thailand for a visit home (UK) in early April, returning to Thailand at the end of April. If I skip the 90-day report due this week, remove the barcoded slip from my passport, and then just report with a new form 90 days after I arrive back in Thailand, will I be fined for missing the previous report, or will they be none the wiser?

It is in the computer - they only question will be if they check your exit or not. You can send someone to do for you.