The R1a haplogroup became a distinct branch of the phylogenetic tree about 40,000 years ago. This site has a nice map of the migration of early R1a members. They originated in the area north of the Caspian Sea and migrated north into Eastern Europe and east into India and Pakistan. There are fewer R1a members in England, and some posit that they descend from Vikings. Here are some interesting papers that trace population migration into England, and generally support that theory:

1. This paper describes how Norse Viking males intermarried with Ireland, Scotland, and English females. Another paper describes this too.

2. This paper describes Anglo Saxon migration, and mentions Viking influence.

3. This paper also describes Viking invasion into the British Isles.

4. Here is a nice review by Underhill with a map of R1a migration.

5. I've included some notes on R1a and R1a Vikings from the DNA listserv.

Here are some links to websites that touch on this topic:

http://www.davidkfaux.org/shetlandislandsY-DNA.html

http://www.davidkfaux.org/dnaprofile2a.html

http://www.nottingham.ac.uk/~sczsteve/survey.htm

http://www.bbc.co.uk/history/programmes/bloodofthevikings/vikingmap.shtml?ne_england

http://www.dnaheritage.com/masterclass2.asp

You also may want to check these books:

http://www.amazon.com/exec/obidos/tg/detail/-/069111532X/qid=1046472128/sr=8-1/ref=sr_8_1/002-1970103-6218424?v=glance&s=books&n=507846

http://www.amazon.com/exec/obidos/tg/detail/-/0812971469/ref=reg_hu-wl_mrai-recs/002-1970103-6218424?%5Fencoding=UTF8&v=glance