�800 is ludicrous!! Think for something that simple you should be able to get it done for �150.
Make sure the first part recites parties. This should be your mother and (can include you) but should also include the execs if there are taxation implications. Then recite the name of deceased, address, date of death, will, probate and terms of the will.
After the recitals, you come to the main part of the Deed which shoudl say that clause x of the will should be read as follows: (insert new wording).
Then there is the s142 IHTA and s62(6) TCGA statements and the stamp duty certificate (should be category M).
Then the attestation - each party signs and has his signature recorded.