Hey Neil,
Although our consulting firm, Revenue Pulse, has made many custom email and landing page templates for Marketo clients, I would like to point you to a resource called Knak. It allows Marketers to create responsive Marketo email and landing page templates, with no coding required. You can update it with your brand colors, logo and social icons and then get the code to pop it into Marketo. From there, you can edit them just like you would with any other template, using Marketo's WYSIWYG editors.
You can build your first template for free, here at: www.knak.io