Thought it might be useful to share installation advice for fonts from Optimo.ch as it's taken a while to work this out. It's probably a basic process for anyone that knows what they're doing. For this, I'm using Plain Medium.
Package of files from Optimo contains, CSS, SVG and EOT. These are be added to a (new) folder called 'fonts' within: public_html/wp-content/themes/lay/. You'll need to access your domain to add these.
Add this to 'Lay Options > Custom CSS and HTML' in both Desktop and Mobile:
src: url('/fonts/PlainMedium-Regular.eot'); /* IE9 Compat Modes /
src: url('/fonts/PlainMedium-Regular.eot?#iefix') format('embedded-opentype'), / IE6-IE8 /
url('/fonts/PlainMedium-Regular.svg#svgPlainMedium-Regular') format('svg'); / Legacy iOS */
Add a new webfont in 'Lay Options > Webfont'
Font name: (same as font-family above)
Link tag: <link href="/wp-content/themes/lay/fonts/PlainMedium-Regular.css" rel="stylesheet" type="text/css">
font-family: 'PlainMedium', sans-serif;
Head into 'Text formats' and change the Font Family to the chosen font.
New font should be installed across the site.
Hope I've covered everything and of use to someone else.