Firstly, try to learn the songs, once you have picked up a Lessers final rattle, which is far-carrying, (The start of their song can only be heard at close range), it is unmistakeable apart from perhaps Cirl Bunting which should not be an issue around here! The calls are similar but can be separated with practice.
 
Lessers are notorious skulkers, look for them in thick, tall, unkempt hedgerows and bushes and scrub. They tend to move about a lot amongst the bushes and can be difficult to see or approach.
 
Generally, Common Whitethroats are more obvious and do have a song display flight. They will tend to come out of hiding and have a look at you before diving back into scrub especially Brambles. They will then spend time scolding you with a hard call until you move away.
 
Lessers always look cold and grey, whereas Commons are quite rufous particularly in the wings and when freshly arrived. Juvenile Lessers are particularly attractive with a black bandit mask, resembling a small shrike briefly.
 
Hope this helps,
Steve

Hi Phil
 
have you got the Collin's guide?  it's all in there mate!
 
I have a v battered copy of first edition  (courtesy airliners)you can have free if you want.
 
the new paperback edition is only about 15 quid

it will be an eye-opener. A superb distillation of the (mostly) best ID features known, tho the Scandinavian influence on call descriptions means these read oddly to English speakers.  
 
What a change from when I started when many many plates just didn't depict what the birds looked like in life and descriptions were trite, or relied on features that were too hard to see in the field or not diagnostic.  Jonsson was an eye-opener but the real breakthru was the MaCmillan guide. Until then the real secrets were in journals and expensive specialist monographs.  
 
and then came Collins - bow down and worship.
 
btw - I was able to ID more birds once i really learned bird topography!  if you can ever get hold of an out of print pamphlet 'The New Approach to Identification' by Grant and Mullarney buy it - it is a brilliant summation of topography and moult for use in the field
 
still loads of birds I can't ID tho