Party wall separated from exterior wall - ties or not?

[wilful]

Hi
I have a Victorian terrace house, built on a hill.
The party wall on one side has separated from the front exterior wall, upstairs in the corner of the front bedroom, near the chimney breast which is also on that wall. Joists run parallel to the party wall.
There is considerable settlement with a diagonal 'crack' in the brickwork - in places there is a 1.5 inch gap between the bricks where the old mortar has fallen out, and near the top where the roof eaves overhang, there is a one inch gap between party wall and outer wall.
I had a similar crack in the wall dividing the hall and front room downstairs, (that wall is only one storey high), which I repaired by just stuffing the gaps with mortar and replastering, about 4 years ago - the crack has not opened up since.
Is this a safe way to deal with the wall on the other side upstairs, or should I get a builder in to tie it with metal straps and resin? I don't have much money and really can't afford a huge bill right now.
The settlement appears to be aged and there have been a couple of attempts to repair the gap previously, once with mastic and in places, with black mortar (Victorian?)
I'm a bit mystified that the party wall just butts up against the outside wall, rather than being knitted in in some way, but I'm just a DIY'er, so I guess it's not unusual? It appears that at some point in the past, the front wall has moved away (forwards and downwards) from the party wall. The whole house is a bit Tim Burton!
Any advice which can save me money but make sure it's safe is appreciated, thanks.

 

[conny]

I'm not a builder so can't give the best/correct advice to this but I would consider using some big right-angled brackets and try to secure each wall to each other using these drilled and screwed into the brickswork. (DO NOT SCREW INTO THE MORTAR!). Hide the brackets below the plaster to acheive a neat finish.

 

[wilful]

Thanks.
I've been looking at steel restraint straps but they only seem to be for attaching timber to brickwork.
Would just rawlplugs and screws be sufficiently strong to hold the straps in place?

 

[theprinceofdarkness]

I would check the ceiling joists and wall plates in the loft. It could be that the roof is spreading and pushing the walls outwards. If the roof ridge runs in the same direction as the front wall and at right angles to the ceiling joists, it might be an idea to tie them front to back with some 4" X 2" running front to back.
I would just use that stainless steel "heli-fix" rods cut them in half, bend them into right angles, rake out the mortar to a depth of 1" in both walls and mortar them in and fill any big cracks as you go.
Frank