Hi Sarah,
I'm assuming he designed your website for you?
Unfortunately some developers do massively overcharge for standard web hosting reseller packages they may use, or even normal shared hosting.
You should have access to your own control panel to be able to add email accounts. Heart Internet's control panel is propriety and isn't great, but adding another email account is still only a few second job. £90 is definitely too much for that. You can argue he can charge what he likes if you don't want to do it yourself, but if you're not getting the option to do it yourself that's out of order in my opinion.
It's hard to tell what resources are currently available to you, but you can probably get the same hosting for far less from Heart Internet direct. However they're certainly not the best option.
A decent smaller host would happily create the email accounts for you at no extra charge. Heart Internet used to be popular with web designers but their standards have really dropped in recent years and they're now little better than many of the large oversold web hosts.
Unless you have specific requirements Linux hosting will generally be more cost effective, and although as stated earlier Wordpress can run perfectly fine on Windows, Linux is generally the preferred option, as it is with any open source content management system.
To put his quote into perspective our Starter
web hosting package is £3 per month or £30 per year and will allow you to run your Wordpress site with ample resources, a free domain for life, and unlimited email accounts. As we run LiteSpeed webserver and don't overcrowd our servers you'd likely see higher performance from this. Alternatively our Startup
eCommerce hosting package is £10 per month or £100 per year and includes a free domain name for life, SSL certificate, and dedicated IP address, and is optimised to run modern CMSs such as Wordpress and eCommerce platforms.
For two years we offer an additional month free, so it'd only be £190. And without knowing specifically what package you have at the moment, that would likely provide a lot more than what you're currently getting.
So I'd say you're being charged quite a premium for what you're getting, especially if you're not even being given control panel access, in which case what you're getting is worse than if you went direct.
Generally it's going to be far cheaper to go direct to a good web hosting company.
You should also make sure the domain is registered in your name and is under your full control.
Anyway I hope that helps. If you have any questions feel free to get in touch.
Good luck,
Dan