I can definitely attest that taking a benzo as prescribed in conjunction with methadone is completely safe. In fact, Xanax and Ativan are not even "contraindicated" to take with methadone. While many deaths are attributed to recreational drug users who are either opiate naive, and paticularly, methadone naive, these medications can be used safely and effectively together. The key is to notify all prescribers that you are taking them and be honest in the amount. It sounds like u r in the UK so I can't give specific guidance for how docs feel about the two over there in conjunction. But I have been on a stable dose of methadone(120) for five years now with less than 2 dose changes +\- 10 mg and have had no problems with either my clinic or other docs as long as both parties are aware and have an informed consent on file to speak with each other if they wanted to. I had been on pain management and low dose Xanax over five years ago and became pregnant. Then my OB had me switch to methadone and stop the Xanax. I found that I liked methadone for my new life as a mother much better than pain mgmt so I've stuck with it. I only added Xanax back in earlier this year after giving life a good shot without it for a few years. My panic/anxiety just became miserable for me and my family so I went to my primary care doc this time and resumed Xanax at low dose again. My clinic wants to see the bottle every time I get a refill and they make a copy of it. I bet if it was like a script for 200 bars a month they would poopoo it and probably take my privileges away to have take home medication. But to sum up, no its not impossible. It does require honesty and Frequent monitoring by ur clinic and likely the ativan doc to keep ur head "straight".
Unless you care a whole aweful lot, nothing's going to get better. It's not. ~Dr. Seuss