1

Both Flatford (http://www.tate.org.uk/art/artworks/constable-flatford-mill-scene-on-a-navigable-river-n01273) and Dedham Lock (http://www.tate.org.uk/art/artworks/constable-dedham-lock-and-mill-n02661) are very close together. Your nearest station to both of them is Manningtree and from there you can either get the bus (takes about 10 minutes to get to Flatford and it's a further 20 minutes walk to Dedham) or if the weather's fine you can walk it in about 30-40 minutes or so.
Hiring a car from London isn't going to save you any time. The train from Liverpool Street to Manningtree is about an hour. If you take the car it'll probably take you near on that just to get out of Central London. If there are 3 or 4 of you going it will be cheaper than going by train though.

2

If you're feeling frisky you can do the walk as detailed here. I have done it and can vouch for it's quality. Dedham is a very nice village worth visiting as well.

3

Thanks guys. By train then bus/walk seems to be the best option as there's just the 2 of us.

"eurotrash": are the written instructions easy to follow when doing the Manningtree Circular walk eg. are there useful signposts on the tracks?

I remember experiencing quite heavy rain when I was in London in September 2010. Would such rain make the pathways on the walk impassable?

4

I recall that walk being quite straightforward, though I have done a lot of the walks from that website so I have experience following these type of directions. It's difficult to get too lost, since there are roads and villages about where you can ask for directions. But it's not an "official" walk so there won't be signs to guide you.

It was very sunny when I did this one so no issues with mud/water. If it has been raining for days before you can pick up some cheap wellies (rain boots) in London (plenty of shops in Covent Garden). But I wouldn't worry too much about it... the rain in London gets greatly exaggerated. In fact, on average it rains nearly twice the volume in NYC each year as in London, but the former city never gets described as particularly rainy.

5

As a rule (though I realise it's no consolation if you're caught in a downpour) this part of eastern England doesn't get much rain. The wind can be pretty cold, though, so dress warmly.