Can I do a visa run after extending my tourist visa in Thailand, or do I need a multi-entry tourist visa?

Apologies if this has been answered b4, but I need some help/advice. I entered Thailand by air late January on a regular tourist entry with a British passport, and extended my stay for 30 days at Bangkok immigration, running thru until 26th March. I'm not sure whether I can do my 1st overland visa run and extend by another 30 days, or do I now need a multi entry tourist to re-enter the Thailand? If so, where is the best and quickest country to go to to sort it out? If its no longer possible, how and where do i go to to organising an MET? Thanks in advance for any help