Will I require a flue liner, and could you recommend a cowl to go on
the top of the chimney?

Firstly, you clearly don't know anywhere near enough to make a safe
installation, so I hope you will be employing a CORGI registered installer
to do the installation.

However, for planning before you employ them, the Greenstar will require the
concentric flue to go all the way to the top of the chimney and terminate in
a purpose built vertical flue terminal. To do this will require quite a few
flue extension pieces (Greenstar specific), which should be allowed for in
your costings. It will also require the flue route to be direct, as it will
be very difficult (read: expensive and possibly impossible) to put bends in
the flue as you pass it up (or down) the chimney. You may find it easier to
install a boiler that accepts a side by side drainpipe flueing system. These
may be easier to install in the chimney (and much cheaper to purchase).
Amongst others, the Keston Celcius 25 has this flue arrangement as standard,
whilst some other manufacturers have this as an option.

Also, if the other side of the chimney is the side of the house and
distances to various features are appropriate, it may be easier just to
stick the flue straight through the wall. Not possible if the other side of
the wall is your neighbours lounge, though.

Christian.