[Ethereum] ethers.js (node.js) call solidity function that is payable

I understand that you can send ethereum to a smart contract, although when you do this with sendTransaction it doesn't seem that you specify a function on the smart contract when you do this. I don't quite understand how this is useful.
However what I can't find out how to do, is to send ethereum to a function that is set as payable.
I'm using ethers.js and can create a contract, and call functions that are viewable (read only) no problem, but I cant work out how to send ethereum with the transaction to a specific function

export const testContract = async (address, abi) => {
  const wei = Utils.parseEther("0.1")
  const url = "http://localhost:8545"
  const provider = new Providers.JsonRpcProvider(url)
    // Load the wallet to deploy the contract with
  const privateKey =
    "0x123"
  const wallet = new Wallet(privateKey, provider)

  var contract = new Contract(address, abi, wallet)

  const price = await contract.retrievePrice()
  console.log("price " + price) //logs the price set in the constructor when the contract was made (WORKS)
  const testAddress = await contract.isUser(
    "0x456"
  )
  console.log("testAddress ", testAddress) //checks if the given address is a user on the contract (WORKS)

  const gasPrice = await provider.getGasPrice()
  console.log("gas price: ", gasPrice.toString()) //returns the price of gas from the network (WORKS)
  try {
    const addToUsers = await contract.requestAccess({ //call function to request access, from the current wallet (REVERTS)
      value: wei
    })
    console.log("result of sending transaction ", addToUsers)
  } catch (error) {
    console.log("error.... ", error) //fires as the contract reverted the payment
  }
}

Any help as to why calling contract.requestAccess is reverting on me? I can't quite work out how to call a specific function and send Ether.
Thanks!

Response to comment

Ok right, so the function in the solidity contract looks like:

  function requestAccess() payable {
    require(msg.value == price, "Incorrect sum sent to contract");
    _addUser(msg.sender);
  }

I commented out the require, to try that but still reverts. _addUser is from a contract mine inherits from, and looks like

  function _addUser(address account) internal {
    users.add(account);
    emit UserAdded(account);
  }

users is Roles.Role private users; and sits in the parent contract

contract Users {
  using Roles for Roles.Role;

which imports from openzeppelin "openzeppelin-solidity/contracts/access/Roles.sol"