Quick question about deed poll form

[NameChange30]

I'm filling in a change of name deed and there's one part I don't understand. The full form is online here and the part I'm not sure about is at the top of the second page, where it says "IN WITNESS". Here's the wording:

I have subscribed my adopted and substituted
[ ] forename(s) [ ] surname(s)
of (NEW NAME)
and also my said old
[ ] forename(s) [ ] surname(s)
of (OLD NAME)
the day and first year above written.

I'm changing my surname but not my first and middle name. So should I just tick the "surname" boxes? And should I write my full name or just my surname in the boxes for new and old name?

If anyone could advise me I'd really appreciate it! Thanks in advance...

Don't know, sorry, but are you aware you can do it free?

Yes I am doing it for free - just found the form online and am filling it in! I need to send it to the Passport Office to change my name on my passport. I think this is the easiest way, I just need two witness to sign, if only I could work out the stupid form...
(The other option is a statutory declaration of change of name, but that has to be signed by a solicitor - it's only £5 but I would have to go into their office which is a pain.)

Remember to get about 10 copies if possible.

Can you change it from, say, 'Not A Londoner' to 'Not A Southerner' so your first names are on both bits anyway?