as stated before, they do not take previous misdemeanours if the application is made "in pursuit of family life" providing you meet some other conditions. Financial, accommodation, proof of de facto relationship.
you're quite right Martyn. Some ECOs ( Entry Clearance Officers) will even see this as a negative. It is common practice for Thai parents to work away and send money home. UKBA know these things. Again I'm talking from experience.
Big factors are financial stability in Thailand and reason why she will return. If you apply for her to stay 3 weeks, even though the visa is 6 month validity, staying longer than 3 weeks is considered a breach. If this happens I'd advise you to contact UKBA. The staff at Cardiff are excellent and will advise accordingly. Those of you who say it's not a breach I had first hand experience of a visitor being detained by immigration in England for staying longer than her application length of stay. I've also had experience of getting a visa for a lady that overstayed 9 years!!! The reason her overstay was discounted was because the application was made quote" in pursuit of family life". There are full guidelines and visa laws/immigration on Government website. Google UK visa and immigration rules.