I am on a Retirement extension currently living in Thailand with my Thai girlfriend, I have 2 adult daughters in the UK who I would like to visit for a 2-3 week holiday. My girlfriend doesn't work as she is fully financially supported by me, she doesn't own any land but has 3 sons and 2 grandchildren in Thailand.

How easy will it be for me to get her a Tourist Visa to come with me to the UK?

Any advice would be appreciated.

If you and your girlfriend are in a subsisting relationship, and have evidence to back it up, then there's no reason why she shouldn't get a visa to accompany you on holiday.

She'll need to satisfy the Entry Clearance Officer that the visit is genuine and affordable and that she's likely to return home at the conclusion of her visit.

Have a read through the pinned topic which give the basics, though the visa names have changed http://www.thaivisa.com/forum/topic/446602-uk-visit-visa-basics/ also have a read through other posts, then come back if you need further advice.

Evidence of your ties in Thailand together could well be used as a "reason to return".