Fixing a wall plate on a dense concrete block wall

Hi,
I am building a shed using dense concrete blocks (not the light aerate ones). The walls are single leaf and I want to put up a double pitched roof ( approx 20 deg pitch ) over it. I need to fix a 3x3 timber wall plate on top of and along two of the walls. The wall plates will take the lower ends of the rafters and collars. The wall ( and hence the wall plate ) is 4m long.
I will be laying a thin bed of mortar first and then place the timber on it and I have a several long restraint straps to hook over the plate and fix to the wall. But I'm concerned about actually fixing the wall plate to the masonry. I've looked at the Building Construction Handbook ( R. Chudley ) but can't see this detail. Plenty of diagrams showing wall plates but no explanation of how it's kept in place.
Maybe additional fixings are not used for the wall-plate but instead it only needs to rest on the top of the wall and the rafters, collars and restraint straps together keep the entire roof assembly in place?
Anyone done this or knows what I need to do? If fixings are needed, can anyone recommend which ones I should use for dense concrete block?
Cross section view: __ | | < wall plate ==== < bed of mortar ---- | | < very top of the | | < single block thickness wall
Thanks
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In an earlier contribution to this discussion,

Like most building construction, it's held together with something you can't buy at at Builders Merchant - called *gravity*!
The restraint straps are just belt and braces.
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