Basically they are trying it on 
Yes - you have a legal right to obtain a copy of your child's school (and SN) file.
The HT having to be there is their own (nonsense) rule - probably to intimidate / discourage you. He/she will be worried about why you want to see them.
Not surprising that the SENCO did not know of this right to see and obtain a copy - they seem to know less about SN rights than we do.
I didn't go in to school to view my own DS's file. I just wrote a letter to the Head stating my right to obtain a copy and confirming that i would pay the costs involved up tothe maximum permitted which, as I said, was £40.
Quote the Data protection Act 1984:
Educational records
Parents, and pupils who are 16 or over, have had the right to see local education authority (LEA) school records for a number of years. The DPA has now extended this right to younger pupils. There is no minimum age: any pupil who makes a written request to see their school records is entitled to do so, unless the pupil does not have the ability to understand what they are asking for. The right applies to any information produced by a teacher, an education welfare officer or an employee of the LEA. Access must be given within 15 days.
In addition to the general exemptions in the DPA:
Information likely to cause serious harm to the pupil or someone else's physical or mental health is exempt.
Information about a possible risk of child abuse can be withheld from a parent if disclosure would not be in the child's best interests.
Educational records can be inspected free of charge. Photocopying charges are limited to a maximum of £1 for the first 20 pages, plus a further £1 for every subsequent 10 pages, up to a maximum of £50. This maximum applies regardless of how many pages are supplied.
Fees
Straightforward requests are generally handled free of charge. However, if your request is time-consuming, you may be asked to pay a fee, based on the number of hours of staff time spent on the request. A certain number of free hours is normally allowed (usually between one and five, depending on the department) followed by an hourly charge of £15 or £20. You will be told the likely charges in advance, and asked if you agree to pay.