Hi Jason
I’m not familiar with the majority of the London area, but I’ve never heard of any of the big parks having Adder populations. I may be wrong, and hopefully another member will give you a more definite answer.
Healthy populations of Adders can be found in the surrounding areas, including South Essex, although many areas can only be accessed by car. A taxi from the nearest underground station may be the least expensive option.
I will email you in the next few days with more information.
 

Hi Jason,
In England we have a website called RAUK. (www.herpetofauna.co.uk). Try the forum it may help.Regent Park area has its own introduced colony of Aesculapian snakes and although they are often seen i have never found any.The Underground trains should be able to transport you to a location somewhere but not being a Londoner you will need further help from somebody.Best of luck 

Thanks for the help guys! 

I don't think any of the London parks have adders. Sadly, I don't think their presence would be tolerated for long. I believe Epping Forest is indeed the only site technically in London that has adders. You'll have a better chance of seeing some if you can travel out to the Surrey heaths or Ashdown Forest in Sussex, though.

Richard Kerridge 

There are populations in the London Borough of Havering at at least a couple of sites. Unfortunately I've never visited the sites myself to provide any useful details. 

I've only heard of a very isolated and tiny population inside the M25 

Jason

I have just got back home after working away and also some euro herping, I fear it is to late for your trip, but I can give you some locations close to London for berus. If you are coming to London again I can posibly meet you at the Dome(north Greenwich) and drive you to an excellent site nr shoreham that has vipera berus, zootoca vivipara,natrix natrix, anguis fragalis, bufo bufo and rana temporaria.There are other sites close by that have trituratus vulgaris,helveticus and cristatus, rana lessonae,kl esculenta and ridibunda.

The same goes for any one else poping over to the UK for some herping.

Kind regards

Paul 

There is no better place to go for adders than Epping Forest it is well renowned for them - all the native herps are listed in the SSSI designation. In London there are only 4 adder populations and these are not advertised or revealed for obvious reasons.

Some of the plains have peak counts of around 18 or so animals with past counts of over 40 - it ranks highly in the national survey - Make the Adder Count - if anyone has any spring counts of adders (or other vipers if you are in Europe) then I have survey forms for 2010 -

Jon