I would like an affordable (under £1000), well built laptop, but
security is mission critical, and overrides all other factors.  What
do you suggest?

Laptops under £1000 aren't hard to find.  But how secure do you need
it?  From whom do you need to keep the data secure from?  teenagers?
hackers/crackers? industrial spies? Police forensics? or the NSA?
(lol)

Anyways, these are some of my suggestions for FREEWARE that you could use.

For startup (Before loading windows)

Compusec has a freeware to ask for a LOGIN NAME and PASSWORD before
allowing Windows to load.
http://www.ce-infosys.com/CeiNews_FreeCompuSec.asp (freeware)

For your data

Use Drive encryption software that encrypts all data on the hard drive.
http://www.scramdisk.clara.net/
Just search on google for a freeware.  If you can't find one then you
should buy one for around $40.

I use multiple encryption softwares that use 448-bit Blowfish, AES
256-bit, 3DES hard drive etc.

Spyware removal and detection
Spybot Search and Destroy 1.3 (freeware)

Virus scan with automatic update
AVAST! Anti-virus (freeware)
Tredmicro (free search online)

Firewall
Sygate Personal Firewall (freeware)
Zone Alarm (freeware)

Internet (deletes cookies, temp, history, cache etc.)
IE Privacy Keeper (freeware)

Some insightful information.  I did not consider encryption at the
software level much before.  Thanks.