I need to fit a smoke alarm in the loft of my thatched property. Should i go for an ionisation type or combined type. Again I was thinking of one with RF capability. Thanks in advance.

I need to know which grade of fire alarm/detectors are you proposing to fit because this dictates the type of detectors available to use. Grade A,B or C, more expensive will give you more choice than D,E or F.  Checkout Guidance on fire safety provisions for certain types of existing housing to decide which grade you wish to install and I can give a more definitive reply.

 Also check out http://www.safelincs.co.uk/pages/bs5839-6.html?url=bs5839-6&allURL=pages/bs5839-6.html and http://www.firesafe.org.uk/fire-alarms/ which will explain more about the grades and the different types of detector. 

Research indicates that photoelectric alarms provide the best detection across a range of fires and are recommended for detecting smouldering fires which is the type you are likely to encounter in a thatch fire.

Ionisation detectors are better at detecting products of  a fire (Flaming fire) and heat detectors would be far too slow at giving a warning. A CO fire detector, which is different from a CO warning detector, but would only be available for a Grade A,B or C system.

Mixed detectors using CO warning detectors operate at levels that inform you CO has reached levels that are becoming toxic but are useless at detecting fire.

Finally I would think interconnected detectors are a must and RF is acceptable.

I have a problem with the smoke detectors in my thatched roof. When there is a high wind the dust created by the thatch triggers the alarm and the detectors have to be disabled. Is there a way they can be insulated from dust and yet remain effective? 

If you have smoke detectors in your thatched roof area, you could have them changed to heat detectors or rate of rise heat detectors.

If you have smoke alarms in your thatched roof area, you could change them to heat detectors

You should also inform your insurance company of what you are proposing and why.

Anything covering/shielding any detector will impede its operation, and overtime may render it useless. (Said item gets clogged with dust, becoming impermeable.)