Rx4 or Rx3 form required?

Hi, 
I'm trying to purchase a property that has a restriction on it. 
My solicitors and the sellers solicitors are arguing over which form is required in this instance.

My solicitor is asking for an Rx4, but the sellers solicitor will only provide an Rx3.

Can anyone tell me what is needed to remove the restriction below at point of sale?

05.01.2017) RESTRICTION: No disposition by the proprietors of the
registered estate is to be registered unless one or more of them makes
a statutory declaration or statement of truth, or their conveyancer
gives a certificate, that the disposition is in accordance with The
{Trust Name} 2016 Asset Trust, a deed of trust dated 19 May 2016 made between
(1) {Parent 1} and {Parent 2} and (2) {Trustee child 1},
{Trustee child 2} and {Trustee child 3} or some variation thereof
referred to in the declaration, statement or certificate.

For reference, both parents passed away a few years ago and we already have the statement of truth


Any help would be greatly appreciated
Thanks in advance

Hi Jack - suggest you refer the solicitors to our PG 24 re Private Trusts and section 6 on how a form B restriciton can be complied with and automatically cancelled in specific circumstances. 
Practice guide 24: private trusts of land - GOV.UK

Thanks Adam, 
That document makes it plenty clear enough.

Many thanks
Jack