The £4.99 is for the tower unit. The additional £2.00 will be for the screen.
The PC Performance cover is banded, £4.99, £6.99, etc based on the price of the unit. Monitors are £2.00 per month unless the cost over a certain threshold then the cost moves to £2.99/month.
There are many varied opinions as to the value of in house extended warranties or service agreements.(which have been covered in the forum a few times), and you are correct in saying that the manufacturers warranty covers parts and labour in the first year (and possibly longer in some circumstances). you need to decide if the additional services (ie software support, access to in store clinic) are worth the monthly payments.
For an experienced user, software support may be needed only infrequently, but for a novice a 'helping hand' may be needed more often.
Weigh up the benefits vs the cost and make a decision based on YOUR needs, not necessarily on others opinions.
hope this helps,