Maxx,
Have a look at Dunnunda and do a search on my name along with JAR ATPL conversion.
The cost depends on what "small turboprop" you flew. If it's a Kingair/Bandit/Twotter it isn't suitable to directly convert your ATPL because it isn't a JAR 25 aircraft.
Your multi crew time is acceptable so long as it was logged under CAO 82.3 rules. (ie 2 pilots were required by law). You MUST get a letter in writing from your employer stating how the multi crew time was logged and the dates of your employ. This verifies your log book and get them to confirm your hours.
If you have this, you can go 2 ways. For the flying, you can do a JAR 25 type rating, eg Shorts 330/360, Dash 8, 737 etc or you can just do an abbreviated IR (15 hours min) and that will give you a CPL. Now, you can convince employers that you don't need an MCC etc because you have enough multi crew time.
Only you can figure out if the cost is worth it.
The cheapest type rating is the Shorts 330/360 at a minimum of 7000 pounds. A 737 rating is about 15,000 pounds.
To reference every thing, search the CAA website and look for GID 25.
If you do your homework you can save yourself 70 pounds and the risk of sending your log book to the CAA.