Builder with drainage expertise needed

[Silks]

Hello

I live in a basement flat in East Dulwich and need to find a good builder with expertise in drains to do some work for me, including fitting a non-return valve. It’s a non-standard job and not something Dynarod can do. The only drainage company I’ve seen recommend on here in the past is CM Drainage but they promised me a quote and then never got back to me, despite numerous chasing calls. 
 

if you can recommend someone good I would be so very, very grateful. My flat has flooded twice in the past and I’d really like it to not happen again. 

thank you!