Extension, or not to be....

[WTFH]

Playing Devils Advocate, You might receive them on a certain date, but that does not mean a more complex structure does not sit underneath, tied to the deliverables.

I doubt HMRC would see it that way.

[darmstadt]

If you get quarterly and yearly bonuses and have your own office on the client site, then you're not a contractor. You're a permie.
To be a contractor, you work because you have signed a contract to do the work. Without the contract, you're just an OR.

Nope definitely contractor. They wanted me to be permie but as they're based in the USA and I'm the only person who isn't then the paperwork hassle would be too much so I just send them an invoice every now and then, normally every couple of months as there isn't too much to do at the moment which is why I have two other contracts, for the work performed and for any expenses incurred. Quite a few of the other employees are not permies either and work on other projects at the same time plus also receive the bonuses depending upon how well the company has done, just a nice thing to do as we're all in it together (there's only about 7-8 permies anyway in a company of about 25 people)

[LondonManc]

You might be better going as associate and classing them as associate retention fees.