First of all as with any defect you MUST inform the house builder in writing without delay.
You have a year left of the two year warranty and in the next 8 years drainage issues may be covered by the Zurich Warranty as it is with the NHBC.
Point of note: Zurich no longer do new home warranty. They will honour and underwrite all warranties already in existence.
I would be very wary of buying a new home that is not warrantied by the NHBC from any house builder.
You may like to ask your self:
1) Why do Bloor choose to not use the the NHBC warranty?
2) Why Zurich decided to get out of this market?
3) Why Bloor use LABC warranty mostly now.
Storm and foul (sewer) waste water systems are designed to water authority regulations and standards and fully inspected by them at every stage during construction.
In fact it is worth noting that water authority inspectors are generally right jobsworths, insisting everything is perfect before they will sign it off.
Mains sewers, especially in adopted (Section 38 agreement) roads are covered under a Section 104 agreement.
This means that at a certain time, the sewer is adopted by the local water authority in your case Severn Trent.
It is doubtful that the sewers are adopted if the development is not 100% complete, even then the house builder is normally required to put right any defects for a period of time (2 years I think)
I must say that Bloor's attitude to this is dreadful.
No one buying a new home in the 21st centuary shoudl be flooded by raw sewage due to heavy rainfall.
The systems should be designed so storm water drainage is completely separate to sewage, with storage culverts if required, to prevent this happening.
However, I feel that the water authority will be your best bet in getting this resolved as it is clear this could happen again.
If Bloor have failed in any respect it will be in Severn Trent's own interests to get it put right before they take over full responsibility.
They are also big enough to take legal action against Bloor should it be necessary.
If this has happend to me I wouldn't want to live there anymore and your home could be difficult to sell.
I doubt Bloor will do the honourable thing and buy the house back from you but you could ask.