Sewage in my dishwasher

Sellins

To cut a long story short.... 
I have sewage only pipes which join a combined sewer across the road. When it rains (badly) the water and sewage backs up my drains and pumps up the drain under the kitchen window. Itgets up the dishwasher outlet pipe and also flows down the side path into the back garden. 
Severn trent have done the minimum and have labled it a "hydrolic overload" event.
They could simply put a no return valve on my sewer, but won't until they know where the water will go next. Does this sound reasonable? Can I expect a time scale for action? 
If I get a no return valve, can i charge them for it?
My sewers are made for water from a bath, etc. This volume of water will be detrimental to them I would imagine, but damage is my responsibility.
I am in poor health,  immunosuppressed , and the advice re the dishwasher is to not touch it and leave it until they come out. It took them 5 days to come out last time.
I would really like to know what I should expect, rather than what ST tell me i will get and is there any organisation that can put pressure on them to fix the problem?
Thanks 

elsien

Start off with a formal complaint making it clear that’s what it is.
https://www.stwater.co.uk/content/dam/stw/tier2_helpandcontacts/customer-contact-complaints-procedure.pdf

also wondering if you could get your local environmental health involved?

Sellins

Hi, I am on tier 2 of the complaints procedure. It is hopeless. Just a layer of people who know nothing and just pass messages back and forth.
I had wondered about environmental health. I will give them a try if I don't get some timescales out of ST tomorrow. I gave them a week.....

DullGreyGuy

You could put a no return on the dishwasher alone, at your own expense, but most dishwashers have one built in already these days. Obviously blocking it from one place just means more black water is just going to come out somewhere else if it cannot get into the dishwasher so its technically a bit of a gamble as where it goes next may be worse. 

Sellins

Hi, i just looked into that. There is no sparepart available for my dishwasher so i guess there isn't one. There is "pipe flap" non return valve that perhaps i could fit, but I don't know where i would put it. I guess even a small angled piece of pipe where the foul water goes into the drain might help as it would stop the water being pushed upwards with force.