1. He decided to cut the hedge to suit himself and without discussion or agreement as to when or how.
2. He's entitled to do that, but cannot make you a party to a contract with someone disposing of the clippings. If it's £100 or £1.50 or £1,000 - no matter, his problem and he's definitely not being cheeky he's taking the proverbial and trying it on.
3.. The clippings are your property and he should have discussed this with you before cutting, in case you wanted to have the clippings to dispose of yourself.
4. The easiest way to dispose of clippings belonging to a neighbour is obviously to speak about it and hand them back over a boundary fence. Not possible perhaps because of the height of the hedge, but they remain your property so prior discussion was needed with you.
5. As it is your hedge, which he has trimmed on his own side before, if he suddenly has a problem with it, it's likely to be because he wants every available bit of space to park a caravan etc.,
6. A "normal" 2 metre hedge in a suburban garden can easily be turned into a much higher boundary hedge if it's properly maintained and does not affect neighbours' right of light or cause damage to their property. A hedge does not have to be an enormous height if it is clearly preventing light to a neighbouring property and complaints can be made to the local Council.
7. Cutting back of branches and leaves etc., which protrude from your hedging into a neighbouring property is perfectly acceptable for a neighbour to do at any time - except in the nesting season for birds. If you or your contractor normally cut the hedge, you can offer to have the neighbouring side cut at the same time - with you covering the cost of that as it's your hedge. Normally neighbours agree as to who will have the responsibility for the other side or agree an access time for a contractor to do the work.
8. Becoming involved in any kind of legal dispute will have to be revealed if either of the parties sell their homes. Not only is it a bad idea to row with neighbours, it's an even worse idea to threaten legal action since this may affect any future sale of either property.
The major problem with hedging such as Leylandii is that too often the little trees are planted and then left. For everyone who plants and cares for such hedging, cutting and pruning regularly to enable a green and frothy hedge to frame their garden, there are vast numbers of people who just plant them, like many other trees, and ignore them until there is no longer a hedge, just a row of extremely tall and ugly trees, which are usually brown and bare at the bottom. Affording no privacy; they grow too wide - take up huge areas of suburban gardens and are spiky and unattractive and mostly dead looking.
You owe him no money because you cannot be forced to pay for a (probably fictitious) bill for work which did not have your prior knowledge or agreement. You can offer to have your contractor cut his side of the hedge next time, or simply announce that it's your intention to replace the trees with fencing. He would have known this before engaging in any work if he had just had the courtesy to speak to you about the work beforehand. (Don't forget to smile!)