I intend flying into Bangkok on a Single Ticket for various reasons, after that I will be using Return Tickets originating from Bangkok which means I will be arriving in Thailand in the future with no flight booked back to the U. K.

will this be problem with Immigration, I have a 'Retirement Visa' ?

I assume with retirement visa in Passport then No.

I had problems flying the return portion on Emirates of a Bangkok London Bangkok ticket about 10 years ago but had no visa at all. I was allowed on after station manager intervened but had to sign disclaimer.

If you have a visa you don't need an onward flight journey. If you have an extension of stay you don't need an onward flight journey.

So you are fine.

Oh one thing you do need to remember is to get a re entry stamp.