Hi,
idle thought in passing:
Most older houses have chimneys which are no longer used. How about a core drill into the stack in the loft, and fit a bathroom type fan. This would vent the hot air in the loft via the chimney, which would give additional 'pull' - you might not even need the fan :-) Coupled with soffit vents on the north side you could have your own loft cooling system. This avoids anyone climbing on the roof and shifting tiles to fit a ridge vent.
Only downside is the bits of brick falling doen the chimney as the core drill breaks through .
Have fun Dave R
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