So, what's the best upgrate plan? I guess the 'no expense spared'
solution would be new land drains plus a Klargester biodisc or similar.
I.E. roughly £9k based on recent quotes. However, any suggestions for a
cheaper solution

Nah. Just go for the Biodisc. You won't regret it. Septic tanks are crap!
They may have worked 50 years ago when people couldn't afford soap, but they
don't now.

Christian.