Customised form not validating

Ok, basically a client wanted to customise the look of a form beyond what the FS plugin allows: placing the fields into two columns, adding tables between fields and other crazy stuff.

I knew the plugin didn’t lend itself to that level of customisation but I thought I’d try this:
– creating the form in the plugin’s settings
– generating the form using shortcode
– copying the generated HTML, customising the DIVs as I liked, then pasting the new code into a custom page template.

This sort of worked: the form looks the way I want and it sends mail OK provided you fill it up correctly.

Unfortunately, validation doesn’t work i.e. if you submit an incomplete form you get a screen refresh without the missing fields being highlighted and you need to start filling it all up again – I should have seen that coming, since without PHP variables in place where would the error messages go?

I feel a bit stupid but now I’m wondering if there is a quick solution i.e. some PHP code I can place before each field to allow validation to take place correctly and for the error messages to pop up in the right places, without the form being reset.

Sorry if all this sounds pointless, I know if would be much easier to either 1. display the dynamicaly generated form as it is or 2. create a custom PHP form but 1. the client wants it that way and 2. I am hopeless with PHP code.

Any help would be appreciated.

https://wordpress.org/plugins/si-contact-form/