i messed up the dates on my o visa and only obtained a 15 day permission to stay on my last visit to Mae Sai.......i leave thailand on the 15th April to return to the UK and have worked out that another 5 trips to Mae Sai for a 15 day permission to stay will see me thru to the end....as i live in CM this option would suit me better than a run to Vientaine with the added expense and hassle............will i have any problems at immigration because of too many back to back 15 day visas?

Under the rules, those stamps are unlimited. However, it is possible an officer may question you about the purpose of your stay in Thailand.

The only problem I would see is how many pages do you have left in your passport? 15 I would hope?

Your taking a chance on this. Go to Vientiane and get a tourist visa they are free until 31 March.

I will get my verification letter regarding my British state pension from the British Embassy but I am also in receipt of a small Swedish state pension. Do you think I would have to get a seperate letter from the Swedish Embassy to cover my Swedish pension or must the verification come only from one's home Embassy?

Ask UK Embassy if they can include if you want but believe only one Embassy letter (yours) is allowed. If small amount probably not worth the effort if UK does not want to do.