Hi - By the feel of it your report is a Landmark Envirosearch Residential. These are the top reports in terms of detail and scope. Without going into depth, as I don't have time at the moment, most of your concerns are quite minor and are not going to influence the property value or the chances of a contaminated land designation being given to the property (which would be a problem). Incidentally, your report insures you against this event now that you have paid for it.
4km from a well is a long, long way and your house is not going to be affected over this distance unless there is a major event. Moderate ground instability is based on the type of soil and rock on which the house is built, it is not a comment about your actual property. If you have at least a basic survey, which you should, you will cover any property specific stability problems and be covered under the surveyors insurance. Powerlines and phone masts are a question of taste really but a LOT of properties are within the shadow of one or both, particularly in urban settings. If you think they are going to do you harm (which some studies suggest) then you will need to rethink, but their presence is really not sufficient to influence the house from a marketability perspective. You have misinterpreted the issue of the park. The environmental constraints are a good thing for the park and for you. It means that the park acts as a receptor to pollution or is in an area recognised for this sensitivity and designated as such under a scheme (there are quite a few). It will enjoy controls that are not present everywhere and there should be tighter controls on the construction of potentially undesirable sources of pollution nearby.
The flooding is worth a closer look. If i understand you, the flooding concerned is surface water flooding and your house is in the 1:200 year risk area, right? This is a complex area, but surface water flooding is usually much less serious that river or coastal flooding. Simplistically, surface water flooding happens when it rains a lot and the water cannot get to the rivers (via drainage ordnance) fast enough. It is normally the result of dense development which creates lots of impermeable surfaces and causes rain to collect. Don't worry about the 1:75 year designation in the high street. The important designation is the one that affect the house (the three bands are quite distinct). The map you have in the report is generated by computer algorithm and it may or may not be the same one that the insurance company uses. Attitudes to surface water and groundwater flooding vary by insurers because the flooding is always modelled by computer and is not considered to be entirely reliable. The thing to do is to phone an insurance company and get a quote. Tell them that you have had the report and that the 1:200 year surface water flooding is on site. I suspect that they will be reasonably philosophical about it, to be honest. I have seen plenty of those reports where the whole page is blue (e.g. Lincolnshire).
Jonathan suggest that you ask your lawyer, but I'm afraid this is unlikely to do much for you. The reason that you have paid landmark fifty or sixty quid is that the lawyer doesn't understand these issues and doesn't want to put his PI insurance on the line by pretending that he/she does. The lawyer will look at page 1 where is says 'Passed' and he/she will file the report - which is precisely what they should do.
There used to be a user guide for the report, which might be helpful. I have found one, but i cannot post links because i have not made enough posts. Google envirosearch_user_guide.pdf and you should find it.
Hope this helps,
