A sponsorship declaration essentially says what you would be saying in the letter but it would be drafted by a solicitor and sworn by you in front of the solicitor. They generally carry more weight with the immigration officials than a letter.
It would cost around £50+Vat from a immigration solicitor in your area to draft.
The points you should both reinforce in your sponsorship declaration and her statement in the application are as follows:-
1. You both intend for the visit to be no more than 6 months
2. You are both aware that she is not allowed to transfer in to any other visa category whilst here on a visitor's visa. You are aware there is non flexibility in this whatsoever.
3. You are aware that any overstay on her part could potentially prejudice any future applications she may choose to make in the future (although in fact they are not allowed to reject on the basis of an overstay alone in the case of fiance/spouses visas) and she would not wish to affect her future rights in this way.
4. You are both aware that any form of deception in this matter is serious and again could prejudice future applications and that neither of you would wish to do so.
5. You are both aware that working (paid or unpaid) is prohibited under a visitors visa and she has not intention to do so, again because of potential prejudice to her future rights. You should also both submit bank statements showing that you have access to sufficient funds to enable her not to work. In you case this should also show proof of your accommodation (eg. Land registry official copies, or tenancy agreement) indicating that she can stay without pay money.
6. You should also state that you will cover any unforeseen expenses during her stay.
If the application is rejected then you can always appeal it and your evidence will get a much fairer hearing on appeal and, if necessary, you can instruct a solicitor to act in the appeal.
I'm afraid it's a product of there being considerably more Australian overstayers in the UK than anywhere else in the world.
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