I am doing the same ,came on a 60 day multiple entry that last 6 months so every 60 days you have to leave and returned and get 60 days each time, this can be done right upto the last day of your multiple entry visa that lasts 6 monthes then you will get another 60 days so you can get at least 6 months easily, even more, it costs £150 on line from Thai embassy in London.
This may look like the best option but buy the time you have paid £150 plus a boarder run every 60 day costing more money and being a pain ,I decided to get a 1 year retirement visa which was sorted by an agent quickly and simply,you don't need to have the 800,000bht in the bank the agent sorted that for me,in the long term it cost less and a lot less hassle .
If you want to know anymore detail Message me.
My retirement visa has to be renewed each year but the agent sorts it all out for
bht(£370) they will sort you a visa for a year without the need for all the checks or money in the bank, they will even sort you a bank account out that is required for the visa for additional cost,you will also need a Thai drivers licence after 6 months,don't forget the international drivers permit from UK to allow you to drive in the first 6 months on your uk licence.
You might get 60 days as it is changing from 30 to 60 soon.
Sorted my partners visa out last year ,first time we have applied lucky I have a good pension had both flights booked and paid for, the application was approved she has a daughter that she was returning to Thailand for. Done it ourselves without a problem.
I got mine from the Post office just before I left on the 11th of April.
When you get you IDP please check you have the correct one for Thailand, last year they gave me the wrong one which made me illegal when driving ,don't notice until this year when I was given the correct one.