MM wrote:

Does anyone have any idea what such a plant would cost to install in
2017/18?

Based on no experience of these systems at all, this one seems to cater
for 30-600 population, which might be oversize for 40 houses

http://www.ukseptictanks.co.uk/sewage-treatment-plants/condersaf50

£11K plus whatever earthworks, plumbing and wiring are required.