Thanks for your reply.
Your husband can apply for a 6 month visitor's visa as a general visitor. To visit as such he will have to show that:-
1. He intends to visit the UK for no more than 6 months (statements)
2. He intends to leave the UK at the end of the 6 months (statements, possibly evidence of continuing residence in the Seychelles (eg, tenancy, statement from employer)
3. He has enough money to support himself whilst in the UK during this time without needing any help from public funds.
As you suggested you should also write a letter confirming you wish for him to come here for this period and he should enclose this with his application.
It is very important that he does not jeopardise any future application he would wish to make for a spouses visa by overstaying his visitor's visa. Also he cannot apply for a spouses visa from the UK whilst here on his visitor's visa, if he decides he wishes to apply for a spouses visa then he will have to return to the Seychelles and make it from there.
He can apply online for a visitor's visa (or by application form) from the UKBA's visa website.
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