Thanks for your furtehr question
You cannot change the years on which HMRC have estimated on - as these were created in your absence to file tax returns so I am afrais in themselves you have to accept the figurework used even if they are greater than they should have been due to absences of work etc
BUT if the estimations used (which should have been created using previous tax years as a guide, IF, these are excessive when compared with those previous years - then you have grounds for appeal.
So for example. lets say your earnings for 2005 were £18,000 and for 2006 were £17,500 then you would expect the estimate for 2007 to be around £18,000 but if HMRC then estimated £40,000 - you can see this would be an excessive estimate. Its on that basis you would have grounds for appeal only
You cannot appeal if they estimated £18,000 but for that year you only earned £10,000 as it was then up to you to report the correct position but for them to have estimated based on the example above - would not be excessive compared to earlier years earnings.
I hope that's clearer