Road bike - sell whole or as parts?

Sounds to me like it's still very much a useful bit of kit.

A snapped cable is nothing really. If you've had it a few years and the cables are getting naff, buy a new set of cables all round (the shimano cables are amazingly slick - but you don't even need to buy expensive cables). Only takes an hour at most to do a full inner and outer cable replacement yourself and theres nothing like the feel of slick shifting new cables!

As for wheels, I'd buy a cheap set of wiggle/CRC own brand wheels Cosine or Prime http://www.wiggle.co.uk/cosine-32mm-alloy-clincher-road-wheelset/ (not currently in stock but the Cosine brand wheels always seem to be on and off the site often).

Then stick some mudguards on and you've got a second bike to use over winter or to go to the shops (or commute)!

You didn't include the option - although others have alluded to it - of getting the parts and fitting them yourself. If the rest of the bike is in reasonable condition then it's probably the most advantageous route. I doubt you'd recoup much more than those repair costs were you to sell it secondhand, whether in whole or parts.

So... you like the bike, and £200-£300 could get you some very good wheels to make it even better - I guess you'd be looking at £1500+ for a new bike which is as good.

Cables are cheap, consumable items as mentioned elsewhere.

Bicycles aren't complicated and should only take a couple of hours to overhaul. If you're not confident, can you bribe a friend with booze, etc? If you don't already have them, you could learn some useful and satisfying skills.

Gear cables are cheap, a new wheelset perhaps not so. Personally, I'd keep the bike. The maladies are easy fixes.

Upgrade, upgrade and upgrade some more.

Do you own a winter bike? If not repair it with some cheap parts, stick on some mudguards and you've got yourself a winter hack.

Not sure what the current value would be, but almost certainly not as much as you want. Keep it as your home turbo bike.

As Vejnemojnen suggests selling a while bike is easier than having bits lasting around for a while.  

If you do that mileage then I expect you already have it serviced or you do it yourself.  If you like the bike get it fixed and ride it.  Perhaps keep as a spare. 

Of not sell it whole and decide whether you want to spend cash putting it right first or if that's cash that could go on the new bike.  

? cabling costs around.. £20-30, a new set of zonda c17 wheels around 300.

service costs? in my country, a whole overhaul (changing bb, truing wheels, changing whole cabling (outer-inners), fitting new chain-cassette, setting up brakes-shifting, re-greasing every bearing (wheels-headset) costs around 20-25, depending on where you live (costs more in the capital, but here in smaller towns, a complete overhaul only sets you back with 20-25 gbps.)

I'd suggest selling the entire bike btw. with splitting, you'll always end up with half the bike unsold..