I found that PHP is server side and there should be an option in the hosting control panel to apply which version of Php to use.
Version 8.1 was recommended.

As member states unless the website is using php scripts (such as this forum) there is nothing to do but if hosting providers want to charge for supporting anything less than v8 then just update it in the control panel to avoid charges.

I hope that this helps.

When Serif's support for forms stopped I created my Contact Forms using "Forms to Go". This no longer appears to be available and I wondered if anyone knew of a similar, more up to date, piece of software that would create the scripting in PHP 8.

I have been using WebPlus X8 for many years and currently have dozens of live websites.  I am like a racing driver, good at driving but little idea what is under the bonnet.
My Internet Provider has recently alerted me to the fact that my websites are using PHP version 7.4 and I have to, from April, use the current version, 8.2.
I have had a look under the various setting of WebPlus but I cannot find anywhere PHP or what version is used, is mentioned.
Has anyone any idea, if it is possible, how I can begin to use version 8.2 going forward for new websites and if I can convert the existing websites to the current version?

PHP is a scripting language. It is not part of WebPlus but something you can use with WebPlus to do things WebPlus cannot do. 
So unless you are using PHP scripts, you don't need to do anything.