Do i just turn up at the airport or do i have to tell the airline
that i will be taking one on board. Anybody got an advice on what to
do or what not to do.

I would have thought that you have to tell the airline. Tey may or may not charge you for it as well. If it is a brand new bike you might want "dirty" it up a wee bit just in case you get stopped by customs.

check with the airline. Can't hurt to let them know.

Could it be dismantled a bit and put in another suitcase? Buy a cheap suitcase, reinforce with cardboard from the bike shop….

smurf

Are you talking about buying a bike out there cheap and bringing it back? If so take your bike bag out with you with a scrap bike in it, ditch this in the US then bring the new one back. They never check/record the contents of your bike bag so you took one out and brought it back, simples.

Post the receipts manuals etc back to the uk and ride it a bit in the us, even on road to make it a bit dirty. Job done.

I am in no way condoning tax evasion and have not done this at all. Not even a bit.

check airline's website – they vary a lot

ANZ, LAX to heathrow, free as long as you don't take more than 2 cases total
AA, think it was $150 each way 😯

One US airline tried to charge Geoff Kabush $200 to take his bike to the Worlds in Australia last week, he came over all cute to the checkin chick and she waived the fee

depends on which airline so check with the airline first
Continental airlines have an embargo on bikes etc certain months of the year (summer being one i think) and the rest of the time you have to pre book