Bascially, I just need to know that if I buy this conventional flue model
stove (efel harmony 5), that I won't need to spend another £xxx amount
having the chimney specially lined.

One of the following websites will point you to a chimney sweep. The chimney
sweep with clean, test and repair your chimney to the required standard and
issue a chimney certificate that confirms that the flue is suitable for your
purposes. They will also state if you need to get the flue completely
relined.

www.chimneyworks.co.uk
www.guild-of-master-sweeps.co.uk
www.nace.org.uk
www.solidfuel.co.uk
www.nfa.org.uk

Due to the small size of the stove (5kw output), it appears that I don't
need
a ventiallation hole either.

Be careful, whatever the regs say, if you have no ventilation, then you die.
If you have leaky sash windows and gappy floorboards on display to the
underfloor void, then you won't have a problem. If you have sealed double
glazing and no draughts whatsoever, you'll get CO poisoning. Don't rely on
the gas fire's inbuilt CO detection. It may not work.

Christian.