Hi Westy... that price is quite good for the size, but all depends on what the current level on insulation is at, within the attic space... Do you know how much that they are topping up by?
With regard to the space between the stud partitions and the outside wall - are they doing the floor insulation & the stud wall (between the joists)? Assume it's fibreglass insulation also? Have you easy access into the space?
Ideally, three detailed written quotations should be sought. So then you can compare like with like... their quotes should detail what is being put in where & how much of...
BER assessment should be independent - but all depends on what you are doing with that - you planning to sell / rent in the near future? I don't know how detailed their own BER assessment will be, could be just the "rating" before and after their work. Proper BER assessments will survey all the house and make recommendations on improvements...
It's a "piece of paper" with a rating on it... a "feel good factor" after the work is done if your house moved up a band... if you want an independent assessment, then you pay for it, and it's good if you are planning additional work in the future...or a necessity if you planning on selling / renting... Personally, I'd prefer to put the money into other things...