Baffled with setting up external Domain to access plusnet webspaces

[#1]

I thought I had this licked, but now I'm just baffled! I tried searching for a similar question, but can't see one.
I've a domain registered with fasthosts.co.uk and I'd like traffic for www.mydomain.co.uk to access my existing webspaces in  www.mydomain.plus.com/htdocs. I'd also like web browsers to show www. mydomain.co.uk in the URL bar rather than www.mydomain.plus.com
The fasthosts registrar has all the usual controls, but I just can't get it to work.
Name servers- I've set that on Fasthosts to ns1(and 2).force9.net
I've tried setting an A record for mydomain.co.uk and www.mydomain.co.uk to  212.159.8.91 which I beleive is the Plusnet webspaces - homepages.plus.net
I've tried setting CName for www and (blank) to point to www.mydomain.plus.com
... but I can't get it to work. Please can some kind soul give me a pointer what settings I need. I have had a pretty lousy response from Fasthosts.
Thank you!
Rich.

[#2]

Have you raised a ticket requesting your domain be hosted by Plusnet?
See Hosting Only here.

[#3]

Thanks for the reply.
No. I migrated over from MadasAFish where it was included, but Plusnet wanted £58 to renew the domain- so although I could keep the webpages hosted on Plusnet I had to move the domain across to another registrar.

[#4]

It  must be done the way spraxyt has said otherwise the Plusnet servers don't know about the domain. When you do that the correct DNS entries are added automatically.
Does the domain appear here with a status of hosted?